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Scientists only became aware of SARS-CoV-2, the novel coronavirus that causes Covid-19, late last year. Most of our immune systems, meanwhile, have still never heard of it. For now, the millions of people who have been infected with the novel coronavirus have to rely on their bodies—rather than a vaccine or targeted medication—to fight off the virus.

The good news: Evolution designed the immune system to do just that. The bad news: Its response isn’t always predictable. The immune system is complex in its own right, and it varies tremendously from person to person. This makes it hard to know how and when to intervene when it’s overwhelmed.