As an epidemiologist, I’ve grown accustomed to viewing the world through the lens of epidemics. Research has shown that it’s not just diseases that are contagious; everything from smoking tobacco to fashion trends can spread amongst people via loose social contacts. And so I’ve been watching closely as Americans deal with a massive, orange-tinged political outbreak: Call it the Trump virus.

Viruses cause epidemics through a number of factors, ranging from transmissibility to the presence of super-spreaders to social network structures to failed control efforts and infection thresholds. The same has been true of the Trump virus. But which virus most closely resembles the spread of Trump’s ideology—and what, if anything, should we do about it now that it’s taken hold of half the nation?

Virus epidemics vary in consequence because of two factors: their innate virulence and society’s response. The last decade alone offers several high-profile, dangerous epidemics that could all foreshadow the outcome of the Trump virus. The 2009 H1N1 flu, or swine flu, frightened humanity as it spread across the globe. But that outbreak turned out to less deadly than feared. On the other hand, Ebola in 2014 initially escaped the attention of watchdogs—ravaging three countries and killing over 11,000 people before it was contained.

In my view, the Trump virus is the influenza of American politics. As we know from our annual flu shots, influenza is a common disease that has infected millions of people each year for generations; it is usually unpleasant but not deadly. Similarly, Trump’s brand of white nationalist politics is not a novel emergence. Race-baiting nativism, economic nationalism, anti-elitism, populism, and grandiose promises have long infected American politics, from former president Andrew Jackson to Louisiana senator Huey Long to Alabama segregationist governor George Wallace. These ideas typically have limited currency, and thus eventually die out.

While influenza is also usually manageable, approximately once per generation—specifically in 1889, 1918, 1957, 1968, and 2009—the flu virus undergoes a major biological shift that unleashes a worldwide pandemic, for which a new human generation is completely unprotected and control measures are relatively futile. Sometimes we get lucky and the novel virus isn’t very deadly, as was the case in 2009. In the worst-case scenario, we get the 1918 Spanish flu—the single deadliest event in modern human history, causing somewhere between 50-100 million deaths.

Like the flu, some form of the Trump virus is always with us. But in the decades since the American Civil Rights movement, it has presented limited danger to the US as a whole. Yet even as Americans again started to believe in the health of their democracy, an evolving Trumpism began to emerge in Europe over the past 10 years. Consider the rise of the National Front in France, Silvio Berlusconi’s multiple electoral victories in Italy, the Law and Justice Party in Poland, and other right-wing populist parties.

The virus used to be anti-black, anti-Jew, and anti-Irish. But in 21st century America, these ideologies would have encountered strong resistance, so it necessarily evolved to become anti-Mexican and anti-Muslim. The virus found a large, susceptible population of mostly white Americans without college degrees. Some in this group had defenses weakened by economic suffering, and the relative health of recent US history meant that they had not developed immunity to his brand of politics.

Moreover, since 1995, many Americans were routinely exposed to Trumpism by renewable sources such as conservative media personalities like Sean Hannity and Rush Limbaugh—that is, people who brazenly foment racial or class grievances. The infection traveled through social networks via super-spreaders like Breitbart News and other conservative, pro-Trump outlets. Vladimir Putin did what he could to make the epidemic worse, using social media and targeted information leaks to fuel the flames. All control efforts, from journalistic fact-checking to Republican establishment resistance, proved futile.

So what do we do now that the Trump virus has affected roughly half of American voters? We can all hope that the consequences will be mild, like the 2009 flu pandemic. But hope is not a strategy. Instead, we should take steps to mitigate the worst damage and detect early signs of catastrophe.

We can prioritize by protecting the most vulnerable, such as law-abiding Muslims crucial to the struggle against radical Islam. We can also limit transmission by contesting Trump’s more absurd proposals by using past examples of protectionism’s sclerosis, the validity of science, and the dangers of demagoguery. Many voters who may sympathize with Trump’s proposals retain the ability to discern whether he will live up to his promises, which he probably cannot, so they may eliminate their infection relatively quickly and become immune.

We can also stop fueling Trumpism’s spread with inadvertently elitist messages, instead uniting as Americans to address the legitimate concerns of many reluctant Trump voters. Those who abhor right-wing identity politics should eschew left-wing identity politics. We can all work to dry up renewable sources of Trumpism—by refusing to pay attention to bloviators like Sean Hannity and Breitbart News, or to their equivalents on the left.

Most importantly, we must work to ensure that the virus does not grow stronger. For example, when Trump inevitably uses a crisis to demand presidential emergency powers, we must urge our Congressional representatives to stop him.

Efforts to contain Trumpism will inevitably be difficult. Many Americans today prefer denial and inertia, despite clear signs of this virus’ danger. But there remains a risk that Trumpism will turn out to resemble HIV rather than the flu. When HIV exploded in Southern Africa in the 1990s, many leaders there ignored or denied it, because acknowledgment was inconvenient or contrary to prevailing ideology. 78 million infections and 35 million deaths later, it is now the new normal.

We don’t yet know whether Trumpism will approach that level of catastrophe. However, history behooves us to take preventive measures immediately and aggressively. Tragedy is only certain in retrospect.