An outbreak spreads as a contagious pathogen finds a new host, and recently the novel coronavirus found a hospitable one in the president of the United States. That the president has been traveling around the country holding large rallies indoors and outdoors and generally not wearing a mask to protect himself has long made him particularly vulnerable to COVID-19 infection, despite the fact that he is the head of state of one of the wealthiest and most medically advanced nations in the world, and that his health and well-being are carefully protected as a matter of national security. Early Friday, he announced his and first lady Melania Trump’s positive COVID-19 diagnoses by tweet, hours after he confirmed that Hope Hicks, who had been traveling in Trump’s entourage, had tested positive for COVID-19 this week.

It may be tempting for Trump’s critics to conclude that he’s gotten what he deserves, given that he has consistently downplayed the dangers of the virus since January and has colossally failed in leading the country’s response to the pandemic at the cost of more than 208,000 Americans’ lives, millions of jobs, and the stability of the economy. He has also peddled false cures, manipulated and silenced government scientists, and openly defied public health experts’ advice in holding campaign rallies.

But no one deserves to get sick; illness is not a curse passed down by angry gods in retribution for misdeeds. It is a matter of biology and, in this case, of public health precautions not taken. That’s the lesson for the president and the American public in this moment — not that we reap what we sow, but that denying basic scientific facts does not make those facts disappear, that dismissing evidence of threats does not make them go away but rather makes them more likely to wreak havoc on our lives and our society.

This is a lesson not just about the coronavirus pandemic, which this editorial board now hopes the country might have a chance of better containing heading into flu season and colder months that drive us indoors. (That is, if the president’s supporters are motivated by his diagnosis to protect themselves from the virus by wearing masks, practicing social distancing, and avoiding people who have tested positive as well as isolating themselves if exposed.) It’s a lesson about the climate crisis, which political and business leaders, including Trump, continue to deny is a threat, continue to downplay as the planet warms, wildfires burn in the West, fierce storms pummel the Gulf Coast, heat waves kill the young and the elderly, farms face historic droughts, and coastal cities flood at high tide. Ignoring the scientific evidence, undermining credible experts and their reports, and carrying on business as usual — opening up more oil and gas drilling in the Arctic and relaxing methane and vehicle emissions rules that exacerbate the climate crisis as Trump has done — will not make these threats go away. It will only make them worse and more likely to cause disastrous consequences for all of us.

The president would be wise to now tell his supporters, those he has in the past called on to “liberate’’ their states from their governors’ advisories on social distancing and, more recently, to gather at crowded rallies to fuel his campaign for reelection, that they ought to be more careful, for their own sake and that of their friends, relatives, and neighbors. And all Americans, Trump supporters or not, Republican or Democrat, would be wise to start heeding the credible predictions of scientific experts about the danger this pandemic poses to their health — and the danger the warming climate poses to it, as well.

Editorials represent the views of the Boston Globe Editorial Board. Follow us on Twitter at @GlobeOpinion.