Wide, unstoppable spread of the coronavirus is exactly an outcome experts are modeling in their worst-case scenarios. They say that given what they know about the virus, it could end up infecting about 60% of the world’s population, even within the year.

Those figures aren’t a random guess. They are informed by the point at which epidemiologists say herd immunity should kick in for this particular virus.

Last week the herd immunity idea blew up in the headlines after UK prime minister Boris Johnson indicated that country’s official strategy might be to put on a stiff upper lip and let the disease run its course. The chief science adviser to the UK government, Patrick Vallance, said the country needed to “build up some kind of herd immunity so more people are immune to this disease and we reduce the transmission.”

Yesterday, the prime minister of the Netherlands, Mark Rutte, struck a similar note, saying, “We can slow down the spread of the virus while at the same time building group immunity in a controlled way.”

But shooting for herd immunity right away would be a disastrous strategy, according to the newest models. That’s because so many people will become severely ill—and a sudden boom in sick people needing hospital or ICU care will overwhelm hospitals. The UK this week signaled it would instead do more to suppress the virus, including discouraging gatherings. Slowing it down would mean health systems could be spared and lives saved, but ultimately the result could be the same. That is, even if the pandemic is drawn out over time, it may still take herd immunity to bring it to an end.

As Matt Hancock, the UK Secretary for health and social care, clarified after criticism of the UK government: “Herd immunity is not our goal or policy. It’s a scientific concept.”

But what exactly is herd immunity?

When enough of the population is resistant to a germ, its spread stops naturally because not enough people are able to transmit it. Thus, the “herd” is immune, even though many individuals within it still are not.

Although it is ghastly to contemplate the prospect of billions being infected by the coronavirus, which has an estimated fatality rate per infection somewhere around 1% (pdf) (that too is uncertain, and the fatality rate of cases rushed to the hospital is higher), we’ve seen evidence for the emergence of herd immunity in other recent outbreaks.

In a simple model of an outbreak, each case infects two more, creating an exponential increase in disease. But once half the population is immune, an outbreak no longer grows in size.

Consider the Zika virus, a mosquito-borne illness that caused a epidemic panic in 2015 because of a link to birth abnormalities.

Two years later, in 2017, there was no longer nearly so much to worry about. A Brazilian study found by checking blood samples that 63% of the population in the northeastern beach city of Salvador had already had exposure to Zika; the researchers speculated that herd immunity had broken that outbreak.

Vaccines create herd immunity too, either when given widely or sometimes when administered in a “ring” around a new case of a rare infection. That’s how diseases like smallpox were eradicated and why polio is close to being erased. Various vaccine efforts are under way for this coronavirus, but they may not be ready for more than a year.

Even then, vaccine makers can find themselves in a losing race with nature to see which protects the herd first. That’s in part what happened in 2017, when drug maker Sanofi quietly abandoned a Zika vaccine in development after funding dried up: there simply wasn’t much of a market any longer.

The coronavirus is new, so it doesn’t appear that anyone is immune to it: that’s what lets it spread and why it can have such severe effects in some people.

For herd immunity to take hold, people must become resistant after they are infected. That occurs with many germs: people who are infected and recover become resistant to getting that disease again, because their immune system is charged with antibodies able to defeat it.