When we get sick, our immune system mounts a response that fights back; our body then has WBCs and antibodies, which give us immunity for many years.
If we encounter the same pathogen again, then our immune system kills it quickly. But this protection doesn’t last forever. Why not?
The genes (DNA) inside the pathogens do not stay the same. In every new generation of bacteria there are random copying mistakes -> mutations.
Most mutations are “silent”. They don’t help, or hurt, the pathogen.
Some mutations are bad for the pathogen: they make it easier for us to kill them.
Some mistakes are good for the pathogen (makes it harder for us to kill them.) In this case, they are now more virulent. That’s good for the pathogen – but bad for us.
The Influenza virus (“the flu”) and HIV (the AIDS virus) both evolve rapidly.
That is why it is difficult to come up with perfect ways of fighting them.
Over time, many viruses evolve to become less immediately dangerous. Why?
There may be an evolutionary trade-off between virulence and transmission.
Consider a virus that exploits its human host more than most – and so produces more offspring than most.
This virus does a lot of damage to the host ( highly virulent.)
From the virus’s perspective, this would, at first, seem like a good thing:
Extra resources mean extra offspring, which generally means high evolutionary fitness.
However, if the viral reproduction completely incapacitates the host, then the whole strategy could backfire:
the illness might prevent the host from going out and coming into contact with new hosts that the virus could jump to.
A victim of its own success, the viral lineage could go extinct and become an evolutionary dead end.
This level of virulence is clearly not a good thing from the virus’s perspective.
However, this not mean that evolution makes all viruses safer, over time. This only means that it makes them less immediately deadly. The above mechanism will not affect the evolution of pathogens that sicken people slowly, and hurt them over many decades.
Plague Time: The New Germ Theory of Disease, by Paul Ewald
“The evolution of antibiotic resistance occurs through natural selection. Imagine a population of bacteria infecting a patient in a hospital. The patient is treated with an antibiotic. The drug kills most of the bacteria but there are a few individual bacteria that happen to carry a gene that allows them to survive the onslaught of antibiotic. These survivors reproduce, passing on the gene for resistance to their offspring, and soon the patient is populated by an antibiotic resistant infection — one that not only affects the original patient but that can also be passed on to other patients in the hospital.”
– University of California Museum of Paleontology’s Understanding Evolution (http://evolution.berkeley.edu).