A global push is on to develop a vaccine to slow the spread of Covid-19, and experts hope several will be ready in 2021. Yet even with one, the coronavirus is likely to remain with us for years, demanding long efforts to find a cure for those who still fall sick.

In humanity’s millennia-long struggle against viruses, prevention with vaccines has been far more successful than treatment with drugs. In fact, modern medicine has come up with a true cure for only one viral infection. For many serious infections, the best approaches are a cocktail of drugs that throw speed bumps in front of the infection.

It’s a lackluster medical armory, belied by the seeming simplicity of our viral foes.

“They can’t live by themselves, they aren’t independent, they can’t process food, take in oxygen, reproduce themselves without the master support system of being the parasite inside a living cell,” said Paula Cannon, a professor at the University of Southern California’s Keck School of Medicine.

So why do viruses give humans so much trouble? Outside of the body, a vigorous hand-washing is enough to kill many. Inside, the immune system’s long memory is enough to make short work of most.

It’s when we run into a new virus that the problems start.

The coronavirus, SARS-CoV-2, is the latest in a procession of new infectious diseases that have surprised the world in recent years. The best hope against it is a vaccine, which can stop infections before they take hold. 

A vaccine is, essentially, a shortcut to immunity. But if we don’t have immunity and get sick, things get more complicated. Because viruses can’t survive on their own, they hijack our cells to multiply. That parasitic dependence makes them hard to treat with most traditional drugs. A virus is so interwoven with its host that it’s difficult to hurt one without hurting the other. SARS-CoV-2 infects the airways and lungs — the very things we need to breathe.

That leaves an unappealing choice, according to Cannon. “I can kill the virus, but I would have to kill you to do it.”

Some vaccines, such as for measles, have created enough herd immunity that the virus can no longer take hold and spread in the population. In the best case, as with smallpox, the shots have driven the disease out of the human host population and into extinction.

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