I was to fly to Boston tomorrow evening, but I'm staying home instead. Just a few days ago, I was thinking all the fear around Covid-19 was silly, but then I caught a very mild cold and, wondering whether that could get me quarantined, sent back home, or barred from travelling, I started looking into the matter, and what I learned changed my mind.

I used to think I was reasonably healthy, and though I invariably catch a bug at conferences abroad, for sleepless nights in tight plane seats and long exposure to unknown pathogens, I'm likely able to survive Covid-19 even if I came across it, which was extremely unlikely to begin with. It's not much more than a flu, after all, right?

Well... It's not so simple. What happens if everyone reasons this way and carries life on as usual? The virus spreads very fast, healthcare units get overloaded, and even people with very treatable diseases fail to get treatment and die.

Avoiding unnecessary physical contact slows down the propagation. In extreme cases, this might even get a virus to die off. Even if we don't go that far, the more we slow it down, the more time we buy to develop treatments and vaccines, and the less we overload healthcare units so that people who need treatment are more likely to get it.

In a lottery, each bet is extremely unlikely to win, but there usually is a jackpot winner with one of those extremely unlikely bets. That's because when you add up all those tiny odds, you end up with a near certainty. That's how propagation of the virus works, too: each person who goes out and gets in contact with people is extremely unlikely to get contaminated, but the more people who reason it is safe and do so, the more likely it becomes for the virus to propagate to them from others who are carriers and don't even know it.

The same goes for gatherings: huge events that are higher risk are cancelling, but smaller ones amount to a much lower risk, so there may be a temptation to maintain them. The problem is that there are very very many of those. If every small gathering reasons that it's safe enough, small odds again add up to an undesirable near-certainty, and the rational and "selfish" behavior of each independent agent leads to a tragedy of the commons.

Avoiding the worse outcomes requires altruistic sacrifices of many who would not even get seriously ill for the sake of the most vulnerable who would. That's why I'm staying home, and I support the cancellation of even small gatherings. To me, it's neither panic nor overreaction, it's the sort of altruism that is the essence of civilization.

Further reading:

So blong...