In January 2015, I spent the longest, queasiest week of my life on a cruise ship filled with conspiracy theorists. As our boat rattled toward Mexico and back, I heard about every wild plot, secret plan and dark cover-up imaginable. It was mostly fascinating, occasionally exasperating and the cause of a headache that took months to fade. To my pleasant surprise, given that I was a reporter travelling among a group of deeply suspicious people, I was accused of working for the CIA only once.

The unshakeable certainty possessed by many of the conspiracy theorists sometimes made me want to tear my hair out, how tightly they clung to the strangest and most far-fetched ideas. I was pretty sure they had lost their hold on reality as a result of being permanently and immovably on the fringes of American life. I felt bad for them and, to be honest, a little superior.

“The things that everyone thinks are crazy now, the mainstream will pick up on them,” proclaimed Sean David Morton early in the trip. “Twenty sixteen is going to be one of those pivotal years, not just in human history, but in American history as well.”

Morton is a self-proclaimed psychic and UFO expert, and someone who has made a lot of dubious claims about how to beat government agencies such as the IRS in court. (In 2017, he was sentenced to six years in prison for tax fraud.) I dismissed his predictions about 2016 the way I dismissed a lot of his prophecies and basic insistence about how the world works. Morton and the other conspiracy theorists on the boat were confident of a whole lot of things I found unbelievable, but which have plenty of adherents in the US and abroad.

Some of them asserted that mass shootings such as Sandy Hook are staged by the US government with the help of “crisis actors” as part of a sinister (and evidently delayed) gun-grab. The moon landing was obviously fake (that one didn’t even merit much discussion). The government was covering up not just the link between vaccines and autism but also the cures for cancer and Aids. Everywhere they looked, there was a hidden plot, a secret cabal and, as the gospel of Matthew teaches about salvation, only a narrow gate that leads to the truth.

I chronicled my stressful, occasionally hilarious, unexpectedly enlightening experience onboard the Conspira-Sea Cruise as a reporter for the feminist website Jezebel, and then I tried to forget about it. I had done a kooky trip on a boat, the kind of stunt journalism project every feature writer loves, and it was over. Conspiracy theorists, after all, were a sideshow.

But I began to notice that they were increasingly encroaching on my usual beats, such as politics. In July 2016, I was walking down a clogged, chaotic narrow street in Cleveland, Ohio, where thousands of reporters, pundits, politicians and Donald Trump fans had amassed to attend the Republican national convention. I was there as a reporter and was busy taking pictures of particularly sexist anti-Hillary Clinton merchandise. There was a lot of it around, for sale on the street and proudly displayed on people’s bodies: from TRUMP THAT BITCH badges to white T-shirts reading HILLARY SUCKS, BUT NOT LIKE MONICA.

Some of the attendees were from InfoWars, the mega-empire of suspicion – a radio show, website and vastly profitable store of lifestyle products – founded by Austin, Texas-based host Alex Jones. For many years, Jones was a harmless, nutty radio shock-jock: a guy shouting into a microphone, warning that the government was trying to make everyone gay through covert chemical warfare, by releasing homosexuality agents into the water supply. (“They’re turning the freaking frogs gay!” he famously shouted.)

President Barack Obama, subject of a number of conspiracy theories, not least that he was born in Kenya. Photograph: Jim Watson/AFP/Getty

Jones also made less adorably kooky claims: that a number of mass shootings and acts of terrorism, such as the 1995 Oklahoma City bombing, were faked by the government; that the CEO of Chobani, the yogurt company, was busy importing “migrant rapists” to work at its Idaho plant; that Hillary Clinton is an actual demon who smells of sulphur, hails from Hell itself and has “personally murdered and chopped up and raped” little children.

Jones and Donald Trump were longtime mutual fans. After announcing his run, candidate Trump made one of his first media appearances on Jones’s show, appearing via Skype from Trump Tower. Jones endorsed him early and often and, in turn, many of the radio host’s favourite talking points turned up in Trump’s speeches. Jones began darkly predicting that the elections would be “rigged” in Clinton’s favour, a claim that Trump quickly made a central tenet of the latter days of his campaign. At the end of September, Jones began predicting that Clinton would be on performance-enhancing drugs of some kind during the presidential debates; by October, Trump was implying that, too, and demanding that Clinton be drug-tested.

Soon after, the US narrowly elected a conspiracy enthusiast as its president, a man who wrongly believes that vaccines cause autism, that global warming is a hoax perpetuated by the Chinese “in order to make US manufacturing non-competitive,” as he tweeted in 2012, and who claimed, for attention and political gain, that Barack Obama was born in Kenya. One of the first people the president-elect called after his thunderous upset victory was Jones. Then, in a very short time, some of the most wild-eyed conspiracy-mongers in the country were influencing federal policy and taking meetings at the White House.

Here’s the thing: the conspiracy theorists aboard the cruise and in the streets of Cleveland could have warned me that Trump’s election was coming, had I only been willing to listen.

Many of the hardcore conspiracy theorists I sailed with on the Conspira-Sea Cruise weren’t very engaged in politics, given that they believe it’s a fake system designed to give us the illusion of control by our real overlords – the Illuminati, the international bankers or perhaps the giant lizard people. But when they did consider the subject, they loved Trump, even the left-leaning among them who might have once preferred Bernie Sanders.

They recognised the future president as a “truth-teller” in a style that spoke to them and many other Americans. They liked his thoughts about a rigged system and a government working against them, the way it spoke to what they had always believed, and the neat way he was able to peg the enemy with soundbites: the “lying media”, “crooked Hillary”, the bottomless abyss of the Washington “swamp”. They were confident of his victory – if the globalists and the new world order didn’t get in the way, and they certainly would try. Just as Morton said, they were sure that 2016 was going to change everything.

Trump’s fondness for conspiracy continued apace into his presidency: his Twitter account became a megaphone for every dark suspicion he has about the biased media and the rigged government working against him. At one particularly low point he even went so far as to accuse his political opponents of inflating the number of deaths in Puerto Rico caused by Hurricane Maria. His supporters became consumed by the concept of the “deep state”, seized by a conviction that a shadow regime is working hard to undermine the White House. At the same time, Trump brought a raft of conspiracy theorists into his cabinet: among them was secretary of housing and urban development Ben Carson, who suggested that President Obama would declare martial law and cancel the 2016 elections to remain in power. There was also National Security adviser Michael Flynn (who was quickly fired), notorious for retweeting stories linking Hillary Clinton to child sex trafficking.

With the candidacy and then election of a conspiracy pedlar, conspiratorial thinking leaked from its traditional confines to spread in new, more visible ways across the country. As a result, a fresh wave of conspiracy theories and an obsession with their negative effects engulfed the US. We all worried late in the election season about “fake news”, a term for disinformation that quickly lost all meaning as it was gleefully seized on by the Trump administration to describe any media attention they didn’t like. We fixated on a conspiracy theorist taking the White House, and then we fretted over whether he was a true believer or just a cynical opportunist. And as left-leaning people found themselves unrepresented in government, with the judicial, executive and legislative branches held by the right, they too started to engage more in conspiracy theorising.

The reality is that the US has been a nation gripped by conspiracy for a long time. The Kennedy assassination has been hotly debated for years. The feminist and antiwar movements of the 1960s were, for a time, believed by a not-inconsiderable number of Americans to be part of a communist plot to weaken the country. A majority have believed for decades that the government is hiding what it knows about extraterrestrials. Since the early 1990s, suspicions that the Clintons were running a drug cartel and/or having their enemies murdered were a persistent part of the discourse on the right. And the website WorldNetDaily was pushing birther theories and talk of death panels (the idea, first articulated by Sarah Palin in 2009, that under Obamacare bureaucrats would decide whether the elderly deserved medical care) long before “fake news” became a talking point. Many black Americans have, for years, believed that the CIA flooded poor neighborhoods with drugs such as crack in order to destroy them.

Trump supporters cheering the then Republican presidential nominee in 2016. Photograph: Jim Watson/AFP/Getty

The Trump era has merely focused our attention back on to something that has reappeared with reliable persistence: the conspiratorial thinking and dark suspicions that have never fully left us. Conspiracy theorising has been part of the American system of governance and culture and thought since its beginnings: as the journalist Jesse Walker writes in his book The United States of Paranoia, early white settlers, including history textbook favourite Cotton Mather, openly speculated that Native Americans were controlled by the devil, and conspiring with him and a horde of related demons to drive them out. Walker also points to the work of the historian Jeffrey Pasley, who found what he called the “myth of the superchief”: the colonist idea that every Native-led resistance or attack was directed by an “Indian mastermind or monarch in control of tens of thousands of warriors”.

The elements of suspicion were present long before the 2016 election, quietly shaping the way large numbers of people see the government, the media and the nature of what’s true and trustworthy.

And for all of our bogus suspicions, there are those that have been given credence by the government itself. We have seen a sizeable number of real conspiracies revealed over the past half century, from Watergate to recently declassified evidence of secret CIA programmes, to the fact that elements within the Russian government really did conspire to interfere with US elections. There is a perpetual tug between conspiracy theorists and actual conspiracies, between things that are genuinely not believable and truths that are so outlandish they can be hard, at first, to believe.

But while conspiracy theories are as old as the US itself, there is something new at work: people who peddle lies and half-truths have come to prominence, fame and power as never before. If the conspiratorial world is a vast ocean, 2016 was clearly the year that Alex Jones – along with other groups, such as anti-immigration extremists, anti-Muslim thinktanks and open neo-Nazis and white supremacists – were able to catch the wave of the Trump presidency and surf to the mainstream shore.

Over and over, I found that the people involved in conspiracy communities weren’t necessarily some mysterious “other”. We are all prone to believing half-truths, forming connections where there are none to be found, or finding importance in political and social events that may not have much significance at all.

I was interested in understanding why this new surge of conspiracism has appeared, knowing that historically, times of tumult and social upheaval tend to lead to a parallel surge in conspiracy thinking. I found some of my answer in our increasingly rigid class structure, one that leaves many people feeling locked into their circumstances and desperate to find someone to blame. I found it in rising disenfranchisement, a feeling many people have that they are shut out of systems of power, pounding furiously at iron doors that will never open to admit them. I found it in the frustratingly opaque US healthcare system, a vanishing social safety net, a political environment that seizes cynically on a renewed distrust of the news media.

Together, these elements helped create a society in which many Americans see millions of snares, laid by a menacing group of enemies, all the more alarming for how difficult they are to identify and pin down.

Let’s pause to attempt to define a conspiracy theory. It is a belief that a small group of people are working in secret against the common good, to create harm, to effect some negative change in society, to seize power for themselves, or to hide some deadly or consequential secret. An actual conspiracy is when a small group of people are working in secret against the common good – and anyone who tells you we can always easily distinguish fictitious plots from real ones probably hasn’t read much history.

Conspiracy theories tend to flourish especially at times of rapid social change, when we are re-evaluating ourselves and, perhaps, facing uncomfortable questions in the process. In 1980, the civil liberties lawyer and author Frank Donner wrote that conspiracism reveals a fundamental insecurity about who Americans want to be versus who they are.

“Especially in times of stress, exaggerated febrile explanations of unwelcome reality come to the surface of American life and attract support,” he wrote. The continual resurgence of conspiracy movements, he claimed, “illuminate[s] a striking contrast between our claims to superiority, indeed our mission as a redeemer nation to bring a new world order, and the extraordinary fragility of our confidence in our institutions”. That contrast, he said, “has led some observers to conclude that we are, subconsciously, quite insecure about the value and permanence of our society”.

In the past few years, medical conspiracies have undergone a resurgence like few other alternative beliefs, and they have a unique power to do harm. Anti-vaccine activists have had a direct hand in creating serious outbreaks of the measles, which they have then argued are hoaxes ginned up by the government to sell more vaccines. There is also evidence that this form of suspicion is being manipulated by malicious outside actors. A 2018 study by researchers at George Washington University found evidence that Russian bot accounts that had been dedicated to sowing various kinds of division during the 2016 election were, two years later, tweeting both pro- and anti-vaccine content, seeking to widen and exploit that divide, too.

Medical conspiracy theories are big, profitable business: an uptick in the belief that the government is hiding a cure for cancer has led people back to buying laetrile, a discredited fake drug popular in the 1970s. Fake medicines for cancer and other grave diseases are peddled by players of all sizes, from large importers to individual retailers. People such as Alex Jones – but not just him – are making multimillion-dollar sales in supplements and quack cures.

At the same time, medical conspiracies aren’t irrational. They are based on frustration with what is seen as the opacity of the medical and pharmaceutical systems. They have taken root in the US, a country with profoundly expensive and dysfunctional healthcare – some adherents take untested cures because they can’t afford the real thing. And there is a long history around the world of doctors giving their approval to innovations – cigarettes, certain levels of radiation, thalidomide, mercury – that turn out to be anything but safe.

Medical conspiracy theories are startlingly widespread. In a study published in 2014, University of Chicago political scientists Eric Oliver and Thomas Wood surveyed 1,351 American adults and found that 37% believe the US Food and Drug Administration is “intentionally suppressing natural cures for cancer because of drug company pressure”.

Meanwhile, 20% agreed that corporations are preventing public health officials from releasing data linking mobile phones to cancer, and another 20% that doctors still want to vaccinate children “even though they know such vaccines to be dangerous”. (Though the study didn’t get into this, many people who feel that way assume doctors do it because they’re in the pockets of Big Vaccine, although vaccines are actually less profitable than many other kinds of medical procedures.)

President John F Kennedy and Jacqueline Kennedy travelling in Dallas, Texas, in November 1963, moments before his assassination. Photograph: Jim Altgens/AP

Subscribing to those conspiracy theories is linked to specific health behaviours: believers are less likely to get flu jabs or wear sunscreen and more likely to seek alternative treatments. (In a more harmless vein, they are also more likely to buy organic vegetables and avoid GMOs.) They are also less inclined to consult a family doctor, relying instead on friends, family, the internet or celebrity doctors for health advice.

The anti-vaccine movement is the most successful medical conspiracy – persistent, lucrative and perpetually able to net new believers in spite of scientific evidence. It is also emblematic of all such conspiracy theories: people get caught up in them through either grief or desperation, exacerbated by the absence of hard answers and suspicion about whether a large and often coldly impersonal medical system is looking out for their best interests. And an army of hucksters stands ready to catch them and make a buck.

The king of dubious health claims is Alex Jones, whose InfoWars Life Health Store sells a variety of questionable supplements. Most of Jones’s products come from a Houston-based company called the Global Healing Center and are relabelled with the InfoWars logo. Global Healing Center’s CEO, Dr Edward Group, is also Jones’s go-to health expert, regularly appearing on the programme to opine about vaccines (he thinks they are bad) and fungus (the root of all evil – luckily, one of the supplements that Jones and Group sell helps banish it from the body).

Group isn’t a medical doctor but a chiropractor, although his website claims a string of other credentials, such as degrees from MIT and Harvard, where he attended continuing education programmes that are virtually impossible to fail provided you pay the bill on time. Until a few years ago, Group also claimed to have a medical degree from the Joseph LaFortune School of Medicine. The LaFortune School is based in Haiti and is not accredited. That one is no longer on his CV.

Several disgruntled Global Healing Center staff members spoke to me for a 2017 story about Group and Jones’s relationship, claiming that the company earns millions a year while toeing an extremely fine line in making claims for its products. “Global Healing Center pretends to care about FDA and FTC regulation, but at the end of the day, GHC says a lot of things that are “incorrect, totally circumstantial or based on incomplete evidence,” one employee said.

Nowhere is that clearer than in the claims that Jones and Group make about colloidal silver, which Jones sells as Silver Bullet. Colloidal silver is a popular new-age health product, touted as a miraculous antibacterial and antimicrobial agent that is dabbed on the skin. But Group and Jones advocate drinking the stuff. In 2014, Group told the InfoWars audience that he has been doing so for years. “I’ve drank half a gallon of silver, done a 10 parts per million silver, for probably 10 or 15 days,” Group said reassuringly.

Group also claims that the FDA “raided” his office to steal his colloidal silver, because it is too powerful. “It was one of the things that was targeted by the FDA because it was a threat to the pharmaceutical companies and a threat for doctor’s visits because it worked so good in the body.”

Edwin ‘Buzz’ Aldrin on the moon in 1969 – an event contested by many conspiracy theorists. Photograph: Rex/Shutterstock

Colloidal silver doesn’t, in fact, work so good in the body; you are not supposed to put it there. The Mayo Clinic says silver has “no known purpose in the body” and drinking colloidal silver can cause argyria, a condition that can permanently turn skin, eyes and internal organs an ashen bluish color. (Jones and Group acknowledge on InfoWars that this can happen, but only when people are using silver incorrectly.) Jones and their ilk complain that they are under attack by the media, the government and some shadowy third entities for telling truths too powerful to ignore.

Unusually, medical conspiracy thinking is not solely the province of the far right or the libertarian bluish-from-too-much-silver fringe. The bourgeois hippie left participates, too. The website Quartz published an astonishing story showing that many of the products sold by Jones are identical to those peddled by Goop, Gwyneth Paltrow’s new-age lifestyle website. And there’s David “Avocado” Wolfe, another new-age lifestyle vlogger, who has called vaccine manufacturers “criminal and satanic” and said that chemtrails are real and toxic. (“Chemtrails” are actually contrails, or water vapour from airplanes, which people in the deep end of the conspiracy pool think are clouds of poison gas being showered on the populace to, once again, make us docile and weak.)

It is only fair to note, however, that these people have been made prominent by the internet, but are also rigorously fact-checked because of it. Jones has been subjected to a very thorough investigation of his claims, particularly since the 2016 election, when his friendship with Donald Trump gave him an enormous boost in public attention. Goop is regularly skewered by doctors, including Dr Jen Gunter, a gynaecologist who takes great joy in wryly puncturing the site’s weirder assertions about vaginal health, such as the benefits of jade “yoni” eggs for vaginal toning.

But it is difficult to figure out whether the two sides balance each other out, whether the scrutiny bestowed by the internet is equal to the new set of consumers it potentially introduces to Goop or InfoWars products. And when people follow the advice of the likes of Jones, it may not only be their wasted money at risk. In October 2017, a nonprofit watchdog group, the Center for Environmental Health, independently tested two InfoWars supplements – Caveman True Paleo Formula and Myco-ZX – and found high levels of lead in both. Myco-ZX is meant to rid the body of “harmful organisms”, and it is one of InfoWars’ most heavily marketed products.

“It is not only ironic, but tragic, when we find lead in dietary supplements, since consumers are ingesting the toxic chemical with every sip and swallow,” CEH CEO Michael Green said in a press release.

“These products are supposed to enhance human health and performance,” Green added, “not lead to increased risk of heart attacks and sperm damage.”

This is an edited extract from Republic of Lies: American Conspiracy Theorists and Their Surprising Rise to Power by Anna Merlan, published by Cornerstone and available at guardianbookshop.com

• Follow the Long Read on Twitter at @gdnlongread, and sign up to the long read weekly email here.