Diseases don't get weaker over time

Since the Covid-19 pandemic began I've seen the idea posited that diseases evolve to become less deadly over time. I want to show who we know that this idea is wrong.

The idea that antigenic drift leads to less deadly variants is a misunderstanding of host-disease co-evolution. It is generally true that if a disease and its primary host organisms have been co-evolving for many generations then we expect fewer hosts to die from the disease.

One interpretation is that the disease has become less deadly. Another interpretation is that the host population as a whole has become more resistant to the disease.

In the first case, the proposed mechanism is that disease organisms which kill their hosts don't spread as much, so the less virulent strains survive. In the second case, the mechanism is the disease kills off all the susceptible hosts before they can breed, and in later generations there only resistant hosts remaining.

So lets do a thought experiment, and see how this plays out.

Suppose there is a disease that initially kills off over 90% of infected hosts. Seventy years later the disease only kills off 10% of infected hosts.

Now suppose we have a time machine. We take a population of hosts from 70 years ago, and we infect them with the modern disease.

• If the modern disease became weaker then it kills off only 10% of time-traveled hosts.
• If modern hosts populations are more resistant, then the disease kills off 90% of time-traveled hosts.
  

Now here is the surprise: This experiment has been done.

In the 1950s the Australian government introduced the South American rabbit disease Myxomatosis to kill off invasive European rabbits (Oryctolagus cuniculus). Initially this plague killed more than 90% of infected rabbits. Today, about seventy years later, it kills around 10% of infected rabbits.

Here's how the researchers did the time travel: Lab rabbits are the same species as Australia 's invasive rabbits (O. cuniculus). With respect to Myxomatosis infection they are  immunologically the same as an Australian rabbit from the 1950s.

So what happened when they infected the rabbits?

Modern Myxomatosis killed off more than 90% of the infected lab rabbits. It has not become weaker. In fact, just the opposite has happened. In the intervening seventy years Australian Myxomatosis has evolved to become even more deadly, picking up new means of killing old rabbits.

The results are very clear: Diseases don't necessarily get weaker over time, rather, host populations adapt.*

* An important point here is that populations adapt. Individual hosts do not. They either die or survive. When people talk about "Covid-19 becoming weaker over time" this really means "all the unvaccinated people who are susceptible to Covid-19 will die." Also, this probably won't work too well, because Covid-19 current kills most people after they have had children, which means that the population likely won't adapt quickly, so it will keep killing old people for a very long time, which means that we have to adapt culturally and technologically.