I am not even sure how to search for an answer on that question -- tried a simple search but results seemed off topic.

The question is prompted from two rather different observations. Long ago I read an article suggesting trees "talked". The sense in which they talk was chemical signals. Trees that were infested with some disease or parasite produces chemicals that were then picked up by nearby trees. The other trees then started producing the defense response to the threat infecting the other tree.

A recent story (via MR) suggests 10% of Boston has the antibodies. So one might take that to mean 10% of Boston was already infected. But the same study says only 1 in 40 are currently infected. These two statistic may be fully inline with antibodies = infection and we just have a lot of asymptomatic people.

However, what if human immune system have some response elements similar to the tress and we respond to signals from others being infected as well as the infection itself?

Anyone know if that is just a crazy thought or not?

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It is not a crazy question at all. In fact it has a variety of answers: I will summarize with a "no," a "yes," and a "no."

1. No, exposure of others to Covid-19 may cause them to become specifically immune to that virus, but it will not cause us to also become specfically immune unless we are similarly exposed to virus components themselves, not just the persons involved.

2. Yes, the fact that people near us are in distress because they are sick will probably cause us, as we too are distressed by the situation, to secrete stress hormone and related responses that will somewhat alter our immune system, for example by raising the count of neutrophilic white cells in our blood somewhat.

3. No, that particular kind of nonspecific change in our stress hormones and related effects on our immune calls is not likely to prevent Covid-19 infection in us.

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Boston has had community spread for almost 3 months. Disease lasts maybe 4 weeks, meaning ~3 cycles of people have already had it and recovered. So 4x as many immune as currently infected sounds like about what you'd expect from that without anything special boosting immunity.

10% actually sounds low to me. Based on my heuristic of assuming the real # is 10x the official #, I would've put it at 20% currently. The study was conducted over a few weeks, meaning significantly fewer were infected or recovered when it started, and only tested people from a few neighborhoods which may or may not be representative of the whole county, so the real number is probably higher than 10%, but still seems like it would be in line with, or lower than, my naive guess.