They mutate from other viruses.
A virus is a little packet of genetic information in a little protein capsule. When people talk about viruses mutating, they mean that the genetic code changes slightly, often when it combines with host DNA, changing the characteristic of the virus.
NSiky: I believe the correct plural is Viræ, not that anyone uses it hehe.
Exactly, that's what I was aiming at. The change in the protein coat would be as a result of the genetic information changing, should have explained a bit better hehe.
Hang on, think the op. knows that viruses mutate but is asking where do they come from? I think they're just THERE like animal, bacteria etc- don't want to get creationists banging on my door but I'd guess were there pretty near the beggining of evolution judging by their simplicity. They're not really "living" but invade and disrupt other cells and when the cells reproduce, the virus is reproduced along with it. Lots of viruses can be carried by bacteria (bacteriophages), but they can exist on their own.
Obviously microbiology not my strong point but it's a hard question to answer!
viridae is the plural used in pathology, but only for technical names. Viruses is fine for everyday use.
The truth is that in Classical Latin no plural form of 'virus' is attested; 'viri' is the genitive singular. Not all nouns ending in -us take -i in the plural; the situation is especially complicated with 'virus', where we're unsure whether it's a second or fourth declension noun, and whether the Romans considered it to be a count or noncount noun. 'Viruses' is, however, English, and should be used.
bored programmers, usually.
Wait - what?
Aah, I shall tell my biology teacher she was wrong. Then again, she did tell us that she only had two lessons of latin before switching to ancient greek, before dropping her classics lol.
Good question actually. In school we've studied the path of evolution (how we all came to evolve from sea sponges etc.) Viruses weren't on the big 'tree' that we researched though. I guess because they're not truly living things, and can only replicate inside host cells, they haven't necessarily evolved in the same way as mammals, fish, reptiles etc.
I suppose it just goes back to the big question of where anything came from, really.