How do diseases like AIDS and HIV form? Why after thousands of years did it only show up in the 20th century?
In: Biology
Diseases are the result of pathogens evolving to infect their host in such a way that they’re most likely to spread and propagate themselves while not being destroyed by the host’s immune system. This is an evolutionary arms war that is as old as time itself.
HIV and AIDS is an example of a disease that does those well. It shuts down the host’s immune response while keeping the host alive for decades so it can be spread to other hosts.
It only crossed over to humans from primates in recent history. In the primate population it would have persisted forever until primates evolved a resistance to it or it otherwise mutated into a different form that altered its characteristics unfavorably. This is the natural progression of almost all diseases.
A virus usually infects a particular host species of animal, but viruses can and often do jump from their primary host species to a new one. When a virus jumps to a brand new host species, that is called a “spill-over event”.
HIV was originally a virus that infected monkeys, but the virus spilled over from monkeys into chimpanzees sometime in the last couple hundred years, because chimpanzees hunt and eat monkeys and thus came into contact with infected monkey blood. Then the virus spilled over again, this time from chimpanzees into humans, sometime in the last hundred years, because humans hunt and eat chimpanzees and thus came into contact with infected chimpanzee blood. So HIV is the result of at least two spill-over events.
Many diseases that infect humans originally came from animals. New strains of Influenza often spill over into humans from pigs or birds. Tuberculosis originally spilled over into humans from cattle. COVID-19 spilled over from bats into humans only just last year.
Side note: HIV and AIDS are not two different diseases: HIV (Human Immunodeficiency Virus) _causes_ AIDS (Acquired ImmunoDeficiency Syndrome). So HIV is the virus that infects you, AIDS is the collection of symptoms you get once the HIV “breaks through”. Note that it can take years for the first symptoms to present after being infected.
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