The Natural History Of The Flu

The ordinary flu is at a kind of idealized equilibrium with humans. It produces symptoms in those it infects, coughing and sneezing, that are necessary to spread the flu to other people. But the symptoms are usually not severe enough to keep home everyone, or even a majority of people, who have the symptoms. The symptoms are mild enough that even those who do stay at home because of them rarely stay at home until the body has completely overcome the flu.

Humans develop immunity to the flu. But the flu mutates into different strains and it is necessary to become immune to each one. Many strains of the flu have a similarity to ones that came before. This is why many people get the flu less as they get older. There is typically one strain in late autumn and another later on in the winter.

But what this says is that humans and the flu have a long history together, and have been continuously adapting to each other by natural selection.

Imagine yourself in the flu's position. You naturally want to spread as much as possible. A virus is not actually alive, it is just bits of genetic code that instructs it's host what to do.

The first thing that the virus instructs the host, in our case humans, to do is to produce many copies of the virus. This is how the virus spreads.

Unlike illnesses caused by bacteria, the virus is not actually alive. A virus cannot move by itself. It relies on something else to move it so that it can spread.

That is where the symptoms come in, in our case coughing and sneezing. The virus needs these symptoms in order to spread to other people. Our bodies try to expel the virus, after creating multiple copies of it, but this is what the virus needs to spread.

Can you see how the virus has reached an idealized equilibrium with it's human hosts?

It has to irritate it's hosts enough to get them to develop the symptoms that spread it, after replicating it, coughing and sneezing. But yet it has to be mild enough so that people who have the flu will not stay home and away from other people, at least not most of the time.

A flu cannot be so mild that it's hosts do not develop the symptoms associated with expelling it from the body, because that is what it requires to spread. But yet the symptoms cannot be too severe, certainly not deadly, because that would keep the host away from other people and the virus would not spread.

The way that the virus must have adapted itself to this idealized equilibrium with humans is through natural selection, over a long period of time. By random chance, due to background radiation and other factors, some virus strains will develop that are more or less contagious than others. Some strains will emerge that are more in harmony with the host, so that they produce less severe symptoms, or less in harmony with the host so that they produce more severe symptoms and can be more deadly.

Eventually the idealized equilibrium that the flu has with us today came to be. The flu has to irritate our bodies enough so that we try to expel it, by coughing and sneezing, because that is what spreads it to other people. But yet those symptoms have to be mild enough so that people do not mostly stay home and so not spread the virus.

Humans developed immunity to the flu but the virus found a way around that, through natural selection, by mutating into different strains. But we can see how people tend to get the flu less as they get older.

This is because the flu has not yet had time to adapt to the fact that human life expectancy has significantly increased in modern times. The virus has not yet developed enough strains so that people past a certain age will get a virus that is different enough from any that they have already developed immunity to for them to catch it.

The great danger with other viruses, such as the Spanish Flu and Ebola, is that they do not have this long history together with humans. They may have jumped from animals to humans.

The conclusion is that the flu was once deadlier than it is today, but not usually deadly enough to "put itself out of business". The flu gradually got milder, through natural selection which favored milder strains because they were more successful in spreading the virus because people were less likely to stay home, and away from other people, if they had a milder strain of the virus.

It may have been that, if a strain of the virus was easier to replicate, that meant it would be more in harmony with it's host and symptoms would thus be milder. The strain would thus be doubly-favored, by being more contagious and also more mild. But a strain that was too mild would die out because the symptoms, coughing and sneezing, are necessary to spread the virus.