A year ago the world was in a panic over Ebola. Now it’s Zika at the gate. When will it end?

The short answer — it won’t.

The next pandemic will not announce itself 10 years in advance as Zika did, nor limit its US impact to two infected nurses in Texas, as we saw with Ebola. No. The next deadly pandemic is more likely to appear as simultaneous outbreaks in multiple countries, and it will be spread rapidly by transportation networks that can have you — or a microbe, or an insect — halfway around the world in less than a day.

What’s most likely to be the next pandemic?

The flu.

You may think “the flu” is just something that makes the kids miss a day or two of school, but an “influenza” pandemic felled 50 million people just under 100 years ago. The “Spanish flu” of 1918 was a novel killer strain, and nothing says that it couldn’t turn up again.

The influenza virus is constantly adapting, which is why we need a new vaccine each year. But these viruses can also swap genes with a bird or pig virus to create an entirely new human bug to which we are not immune. There is a bird flu virus, for example, that does not spread easily between humans but kills more than half of those who do get infected.

And New Yorkers are particularly vulnerable. Flu can be spread by a sneeze, a handshake or the handrail in a subway car. In a megacity, population density makes it that much easier to spread disease and infect the greatest number of people, leading to a national pandemic. Based on the number of Americans killed by the flu in 1918 (675,000), and the fact that our population is now three times larger, a novel strain with the same mortality rate could kill perhaps 1,500,000 of us if it struck today. If the next bug to come our way is a more easily spread mutant of the bird flu that kills 60 percent of its human victims, the result would make the Black Death seem like a day in East Hampton.

“In today’s interconnected world, a disease anywhere is a disease everywhere.”

Moreover, if the 1918 outbreak is the playbook, it won’t be just infants, the elderly and those with chronic medical conditions who are affected. The Spanish flu was particularly deadly among those in the prime of life because it prompted a supercharged — and deadly — response in healthy immune systems called a cytokine storm. Your lungs become flooded with immune cells and fluid, and the inflammation spills over into your entire body, making it difficult to maintain normal temperature and blood pressure.

The threat of a handful of cases of Zika in the United States is real, and the health consequences of infants born with a congenital Zika syndrome that leads to small brains, blindness and deafness will be with us for decades. More importantly, these cases highlight how easily various infectious diseases are brought into the US by infected travelers and how rapidly they can spread, especially in health-care settings.

While exotic diseases like Ebola and Zika get the panicked headlines, we routinely get the same kind of insidious spreading with tuberculosis, measles and new drug-resistant microbes. In today’s interconnected world, a disease anywhere is a disease everywhere.

So far, except for HIV/AIDS which has caused approximately 750,000 deaths in the United States, we’ve dodged the bullet on truly appalling outbreaks. But the risk is real. If not from influenza then from a more deadly mosquito-borne disease or from another deadly sexually transmitted disease, or from some cousin of SARS and MERS that has mutated to spread more effectively. And we can never forget our friendly local terrorist, or complacent researcher. A deadly microbe like smallpox — to which we no longer have immunity — can be easily recreated in a rogue laboratory or inadvertently released.

To prevent the next pandemic, whether exotic or mundane, we need to support investment in our public-health protection system rather than letting it limp along, underfunded and mostly ignored, hoping we can ramp it up when the crisis hits and everybody starts panicking. At a minimum, this requires increased US investment in preparedness efforts, including the creation of a national White House coordinator and an emergency fund. Internationally, we need a new global fund for preparedness and a new United Nations undersecretary for health security to maintain high-level political engagement.

We also need to get beyond our focus on microbes and address our own role in creating the conditions that make deadly outbreaks almost inevitable. As we saw last year with Ebola in West Africa, it is the failure to quickly identify and aggressively respond to small, local outbreaks that allows them to become widespread catastrophes.

Every preventable death is tragic, but the prospect of a flu — or any disease — that could spread to a third of the population and kill 10 percent to 20 percent of those infected should be a call to action.

As further motivation, it may help to simply imagine the horror: Until a vaccine became available, the streets would be deserted and the economy would collapse.

There would be fewer healthy workers to staff public safety and public utilities such as water, power and communications. But the panic would really set in when loved ones are sprayed with bleach, put in the equivalent of Ziploc bags and buried in mass graves.

We simply need to do better. We also need to keep in mind Louis Pasteur’s sobering assessment: “Gentlemen, it is the microbe who will have the last word.”

Dr. Ali S. Khan is the author of “The Next Pandemic” (PublicAffairs) and the former director of the Office of Public Health Preparedness and Response (PHPR) at the Centers for Disease Control and Prevention. He is currently dean at the College of Public Health at the University of Nebraska Medical Center.