So viruses actually mutate by recombining genes from different individuals...
In other words, they have sex!

I've always said that for all practical purposes, viruses are alive.

^ No, this is not an act of reproduction between two strains. Each virus is doing its own thing, which is insert a strain of their genetic code into a host cell so that it can use the cell 'machinery' to make copies of itself. In a situation where 2 viruses are in a host, there is a chance that during the assembly of the copied viruses, a part of genetic code from one virus combines with the other, so when all the parts of the new copy come together, the genetic code of the virus is entirely new.

For instance, in here:
http://www.accessexcellence.org/RC/VL/GG/images/examples_of_viral.gif

During steps 2-4 of the lythic pathway, you can see that different parts of the virus are being made, and it is later that they are assembled. If you have parts of two viruses in a cell, then there is a chance that during the assembly of the viral copies, a part of one virus is assembled into the other (steps 3 and 4)

^That's getting there, but is not entirely the case for flu viruses.
The case you referred to in the pic was a "bacteriophage" which is what most people picture when they think of a virus. that one only infects bacteria and its genome becomes part of the infected host's. It also exits an infected bacteria by destroying it and flooding out.

Flu on the other hand, does not become a part of your DNA genome and doesn't kill the cell to get out. Here's a nice flash presentation that shows what happens:
http://www.nescent.org/eog/influenza_life_cycle.swf

You can notice the many bits of DNA that get into the cell from the virus. Influenza has a multi-unit genome... you can think of it like our chromosomes sort of. When TWO different strains of flu infect one particular cell, the virus that is produced in the end can have some of the bits of DNA from one and some from the other... in essence, producing a new strain which is a sort of daughter of the original two parents. So in a way, I guess you could call it "sex" metaphorically. It's one of the best ways for producing genetic diversity in flu and it's why you have to get a different flu vaccine every year. They try to make their best estimate what will be the common strain for that year. What a pain.

I see, thanks so much for your explanation.

^Doc_M
Yeah, that was what I meant, but in a stupid, sound-bite sort of fashion

Our kind of sex is of course much more sophisticated and reliable, but in both cases the end result is a new individual where the genes are mixed together from both its "parents".

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