Read: The best hopes for a coronavirus drug

COVID-19 is, in many ways, proving to be a disease of uncertainty. According to a new study from Italy, some 43 percent of people with the virus have no symptoms. Among those who do develop symptoms, it is common to feel sick in uncomfortable but familiar ways—congestion, fever, aches, and general malaise. Many people start to feel a little bit better. Then, for many, comes a dramatic tipping point. “Some people really fall off the cliff, and we don’t have good predictors of who it’s going to happen to,” Stephen Thomas, the chair of infectious diseases at Upstate University Hospital, told me. Those people will become short of breath, their heart racing and mind detached from reality. They experience organ failure and spend weeks in the ICU, if they survive at all.

Meanwhile, many others simply keep feeling better and eventually totally recover. Kola’s friend Karan Mahajan, an author based in Providence, Rhode Island, contracted the virus at almost the same time she did. In stark contrast to Kola, he said, “My case ended up feeling like a mild flu that lasted for two weeks. And then it faded after that.”

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Listen to James Hamblin interview Kola and Mahajan on an episode of Social Distance, The Atlantic’s podcast about life in the pandemic:

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“There’s a big difference in how people handle this virus,” says Robert Murphy, a professor of medicine and the director of the Center for Global Communicable Diseases at Northwestern University. “It’s very unusual. None of this variability really fits with any other diseases we’re used to dealing with.”

This degree of uncertainty has less to do with the virus itself than how our bodies respond to it. As Murphy puts it, when doctors see this sort of variation in disease severity, “that’s not the virus; that’s the host.” Since the beginning of the pandemic, people around the world have heard the message that older and chronically ill people are most likely to die from COVID-19. But that is far from a complete picture of who is at risk of life-threatening disease. Understanding exactly how and why some people get so sick while others feel almost nothing will be the key to treatment.

Read: The pandemic is turning the world upside down

Hope has been put in drugs that attempt to slow the replication of the virus—those currently in clinical trials like remdesivir, ivermectin, and hydroxychloroquine. But with the flu and most other viral diseases, antiviral medications are often effective only early in the disease. Once the virus has spread widely within our body, our own immune system becomes the thing that more urgently threatens to kill us. That response cannot be fully controlled. But it can be modulated and improved.

One of the common, perplexing experiences of COVID-19 is the loss of smell—and, then, taste. “Eating pizza was like eating cardboard,” Mahajan told me. Any common cold that causes congestion can alter these sensations to some degree. But a near-total breakdown of taste and smell is happening with coronavirus infections even in the absence of other symptoms.