Origin of sexually transmitted diseases

[Nisse]

We're told in school that the only way to catch an STD is to have intimate relations with someone who's already infected.

If that really is the case, then how can these diseases have started in the first place? There must, logically, be some other way of contracting them.

[negitron]

Mutations. An airborne pathogen has a gene accidentally switched off or switched on or transposes a couple base pairs and, BAM, you've got a blood-borne pathogen, instead.

[Borek]

If that really is the case, then how can these diseases have started in the first place? There must, logically, be some other way of contracting them.

Please elaborate, I can't see a logic conection.

[Nisse]

Please elaborate, I can't see a logic conection.

If the only way to contract a disease is by coming into contact with an already-infected person, how did the first person contract it?

[f95toli]

If the only way to contract a disease is by coming into contact with an already-infected person, how did the first person contract it?

As negitron has already pointed out: Mutations.
In the simplest case one could imagine someone carrying a harmless virus that mutates in his/her body and becomes an STD that can then be transmitted.
In reality it is more complicated than that; but this is the basic idea.

[Borek]

If the only way of getting flu is to contact someone with the flu, how did the first person got it?

Do you see that it is not a problem with STD, but with any contagious disease?

[Astronuc]

If the only way of getting flu is to contact someone with the flu, how did the first person got it?

Do you see that it is not a problem with STD, but with any contagious disease? STDs require more intimate contact than one requires to get the flu. STDs are not airborne, but usually require gential-oral/genital/anal intercourse.

In fact, I have heard sometime in the past, that humans (mainly males of the species) may have contracted venereal disease through intercourse with domesticated animals, e.g. sheep or pigs, then simply passed it on to human females or other males.

Certainly promiscuous behavior will increase the risk of being exposed to an STD.

There is a practical side to abstinence and monogamy. It simply reduces or mitigates the risk of STD.

[tchitt]

There's actually an animal porn industry (google it, if you dare.) so I don't see why it's unbelievable that people didn't pick up these diseases from intercourse with animals. I doubt this is the only explanation for all STD's though... You don't necessarily need to engage in sexual intercourse to catch herpes.

[KingNothing]

I'm willing to bet my ex-girlfriend is the source of most STDs.

[Borek]

STDs require more intimate contact than one requires to get the flu. STDs are not airborne, but usually require gential-oral/genital/anal intercourse.

I think you have missed the point - problem reported by OP is not a problem of STDs, but of every contagious disease no matter how it does spread. Before someone else could be infected, someone has to contract the ilness first. As there are many contagious diseases obviously there must be some flaw in the OP understanding of the problem - and I tried to show it by reductio ad absurdum.

[Nisse]

Thank you to everyone for clearing that up for me (no pun intended).

I only used STDs as an example because that's what first got me thinking about this. I'm not a medical person and the question has been bothering me for ages, and you're the first people to give me a sensible answer, so thanks again.

[Astronuc]

I think you have missed the point - problem reported by OP is not a problem of STDs, but of every contagious disease no matter how it does spread. Before someone else could be infected, someone has to contract the ilness first. As there are many contagious diseases obviously there must be some flaw in the OP understanding of the problem - and I tried to show it by reductio ad absurdum. Unicellular organisms have been around longer than multi-cellular organisms. I was responding to the origin of STD's in humans specifically. It is very difficult to determine the precise origin of each disease.

An interesting book on the subject Microcosmos (http://books.google.com/books?id=VeiwSEETa2wC&pg=PA141&lpg=PA141).

[vibjwb]

This may help you understand how aids was introduced to humans.

http://en.wikipedia.org/wiki/Origin_of_AIDS

According to this it was not introduced by sexual contact