How did STD start? Anyone know their origination?
Question:
My next question is how to diseases start - the type with viruses and bacteria?
Answers:
STDs result from infection by parasites and/or pathogens. All parasites and pathogens require a host to replicate. (I don't say reproduce because viruses are not considered 'alive'; they are infectious agents and cannot reproduce in the biological sense.)
A living pathogen is often a mutation of what had been a free-living organism. Some free-living organisms become dependent on a host in a good way (think lichen - which are usually a combination of algae and fungus) but others are pathogenic and cause harm to the host. The mutant version (that is no longer free-living) is able to take advantage of a new resource - the host - and still survive and reproduce.
Viruses are little bit the same and a bit different. They are only genetic material and a protein "coat". They are not cellular like other pathogens, and so are not considered "living". They also mutate (a lot!) and take advantage of new hosts, but they all require a host to replicate as far as I know (I'm not a virus specialist). Some use plants or other animals as hosts, so I suspect that one that was using a different host and a mutant was able to penetrate a human cell and - voila - we have a new type of viral infection. Some cause STDs, some don't. If you think about it, this is the current fear with the avian flu. Right now it's not a problem in human populations, but the potential for it to infect us is real.
So, unless a person is unlucky enough to be the first host that a mutant pathogen finds (that would be the situation you describe as someone "developing" the infection), the only way for an STD to start in a new person is because they were exposed to a previous person with that infection - that is to say, through transmission by some means. So in our lifetimes (probably not yours, actually, you're too young!) we have only seen one new human STD - HIV - that may have entered the human chain through blood of a primate contaminated with SIV (S=Simian), a virus very closely "related" to HIV. The pathogens/viruses for every other STD out there have been getting swapped among humans for generations, even centuries.
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