Media playback is unsupported on your device Media caption Donald Trump: Clinton "lacks the mental and physical stamina" to defeat so-called Islamic State

In the 2016 presidential campaign, the line between fringe theory and stump speech material is blurring.

In less than a month, unsubstantiated claims about Hillary Clinton's health have gone from chatter on Twitter to an attack line by Republican presidential nominee Donald Trump.

Without offering any evidence, Mr Trump said in Ohio on Monday that she "lacks the mental and physical stamina to take on Isis [Islamic State] and all of the many adversaries we face". Mr Trump, 70, is almost two years older than Mrs Clinton.

Most of the latest reports on her health assert that she never recovered from a blood clot that she suffered in 2012, and is keeping her frailty secret.

Her doctors say she fully recovered after surgery. Last year, in a letter accompanied by test results, her personal doctor Lisa Bardack said "she is in excellent physical condition and fit to serve as the president of the United States".

So how did we go from "fit to serve" to "secret health crisis"? Here's how it happened:

Early summer: Users on Twitter begin speculating about Hillary Clinton's health using the hashtag #HillaryHealth.

4 August: The website Infowars, run by conservative conspiracy theorist Alex Jones, begins publishing stories about Mrs Clinton's supposed failing health. The articles uses a clip of Mrs Clinton joking with an Associated Press reporter in June as evidence that the former secretary of state is "having seizures even while in public". The reporter Lisa Lerer wrote a column debunking the claims. "Where I saw evasiveness, they see seizures," she wrote.

7 August: Conservative blogger Matt Drudge runs a photo of Hillary Clinton being helped by aides after slipping on some stairs with the headline "Hillary conquers the stairs". Mr Drudge directed readers to a story, which claims "The questionable health condition of Hillary Clinton should be a major issue of the 2016 campaign." The problem? The photo was taken in February and news reports at the time showed she continued campaigning after the minor fall.

Image copyright Getty Images Image caption This photo from February was circulating again in August

Infowars also addresses the photo of Mrs Clinton on the stairs. Infowars claims the aide helping her is actually a doctor and an item he is holding in his hand is an injection pen for the drug diazepam, which can be used to treat seizures. (Fact checking website Snopes says it actually was a small torch.)

8 August: The National Enquirer tabloid, which has endorsed Mr Trump for president, runs a story entitled "Hillary Clinton's secret health crisis".

8 August to 11 August: Fox News presenter Sean Hannity runs multiple segments on Hillary Clinton's health on his primetime chat show, claiming that she suffers from seizures. He also cites Mrs Clinton's exchange with Ms Lerer as evidence. (Ms Lerer was never contacted by Fox News.) Mr Hannity brought on medical experts to his show to confirm his suspicions, but the doctors would not concur. On CNN, Trump supporter Jeffrey Lord also repeats the claims.

11 August to 14 August: Several news organisations including CNN and the Washington Post debunk the claims pushed by Mr Hannity and Infowars. CNN media reporter Brian Stelter called Mr Hannity's coverage "reckless".

15 August: Mr Trump says Mrs Clinton "lacks the physical and mental stamina" to fight the so-called Islamic State.

16 August: The Clinton campaign releases a statement saying the Trump campaign was pushing a "deranged conspiracy about Clinton's health". The news release included a new statement from Dr Bardack, Mrs Clinton's personal physician.

"To reiterate what I said in my previous statement, Secretary Clinton is in excellent health and fit to serve as president of the United States," said the doctor.

Image caption The website Heat Street collected images of Mrs Clinton sitting with pillows, adding arrows for emphasis

18 August: The Drudge Report returns to the failing health story with a piece entitled "Pillows for Hillary". The linked article collected various photographs of Mrs Clinton sitting in chairs with pillows, making the case that she was being "propped up".

Later on MSNBC, Trump campaign spokeswoman Katrina Pierson said Mrs Clinton suffers from "dysphasia" without citing any evidence. Dysphasia is a serious neurological condition that affects a person's speech.

Reporting by Tim Swift