The classic biology teacher argument looks like this; “Diseases don’t want to kill their hosts, since they don’t want to die off themselves.” The implication being that diseases like COVID-19 should naturally become more benign as they evolve.
It would seem a better argument for parasites like ticks, fleas and worms with their longer life spans and tendency to stay longer with one host.
Supposing what makes an infectious disease is really it’s transmissibility, it’s ability to actively break out of the host to infect another. If we consider there are roughly 1,400 pathogens capable of infecting humans, among which fewer than 150 have the potential to cause epidemic or endemic disease. Only ~11% can actively transmit to other humans. The implication is it’s far easier for pathogens to infect a human than it is to transmit from a human. Transmission is a big hurdle to disease viability.
Perhaps for many disease causing pathogens to be successful, maximising their attempts to infect future hosts would be synchronous with the host’s immune responses to protect itself. This wouldn’t apply to sexually transmitted infections, we’re considering airborne respiratory diseases like the common cold and vector borne diseases like malaria. The human immune response against the pathogen would also help the pathogen transmit, and would create a stable pattern that could persist for very long periods of evolutionary time. That repetition would be what we recognise as disease.
Let’s consider the respiratory disease whooping cough as an example. Whooping cough takes it’s name from the severe hacking cough it causes followed by a high-pitched intake of breath that sounds like “whoop.” The cough would help clear pathogens from the airway, but the same action would help pathogens spread to future hosts.
For malaria the high fever would normally help the body fight infection but here helps future malarial mosquitoes find the infected host. Likewise, the feeling of tiredness we associate with being ill and may consider the body conserving energy to fight the disease, would also make the host immobile and easier to feed on.
For cholera the body expels the pathogen as a watery diarrhoea, which also helps the bacterial pathogen to infect future food and water supplies and find its way back into future hosts. Interestingly, host infections will naturally cause damage to the host in reproducing, but this isn’t necessarily true for all pathogens. The vibrio cholerae bacteria that cause cholera reside in the intestinal mucosa but doesn’t infect intestinal cells, so here the host would have no obvious need to fight the infection. But virbrio cholerae produces a powerful toxin.
But for this to work the host has to have an incentive to eliminate the pathogen, and to do this the pathogen has to harm the host… which very much undermines the idea that pathogens would necessarily become less virulent over time.