STD's, I've always wondered.... Options

[emberfly]

Where do they come from?

People have said that they come from having alot of sex, but that's probably the most vague answer there is.

They just magically appear after you've done a certain number of people?

Exactly how to they come to be?

[emberfly]

OK, I googled it (thankfully it didn't show any images).

It said STI is the infection, which is much less likely to spread from person to person, like HIV is the infection form of AIDS. The disease form, when symptoms are shown, such as sores or whatever, like AIDS, it is much more likely to spread from person to person.

It also said that the diseases come from having sex with someone who has the disease, and that it is impossible to get an STD from just having alot of sex; you have to have sex with someone infected or diseased.

Unfortunately, it did not answer my question.

It said that diseases are formed from pathogens, Bacteria, fungi, protozoa or viruses, BUT it never said how you get those things inside of you.

You just randomly wake up one morning and, oh my gosh! You have this horrifying infection that will ruin your life?

Please clear it up.

edit

Yahoo answers says that STD's are mutated forms of other viruses, ex: Herpes is a mutated form of Chicken Pox.

It also said that many of the diseases originated from animals.

But still, it did not answer my question, how do you get them?! What do you have to do to get one?! What makes these viruses mutate? What do you have to do?