The most oft-mentioned myth about free software -- aside from the fact that the "free" does not mean "gratis" -- is that the General Public License, the example of copyleft free software licenses, is a "viral" license. It is not, and please stop saying that it is.
Firstly, please acknowledge that no legal document can be viral. The word "viral" is defined to be:
viral: of, relating to, or caused by a virus.
And "virus" is defined thusly:
virus: 1. archaic: VENOM(1)
2. a: the causative agent of an infectious disease. b: any of a large group of submicroscopic infective agents that are regarded either as extremely simple microorganisms or as extremely complex molecules, that typically contain a protein coat surrounding an RNA or DNA core of genetic material but no semipermeable membrane, that are capable of growth and multiplication only in living cells, and that cause various important diseases in humans, lower animals, or plants; also: FILTERABLE VIRUS. c: a disease caused by a virus.
3: something that poisons the mind or soul (the force of this virus of prejudice -- V. S. Waters)
4: a computer program usually hidden within another seemingly innocuous program that produces copies of itself and inserts them into other programs and that usually performs a malicious action (as destroying data).
None of these definitions are applicable to a software license agreement, and it is silly to say that they do.
Of course, I am being (deliberately) over-precise in this regard, and that when people call the GPL viral they are speaking about its qualities, the specific quality that it is an infectious license.
Except it isn't infectious, either.
You therefore have a choice. You can use works distributed under the GPL to create your own software and license that under the GPL, or you can NOT USE the GPL software and use any license you want. If the GPL were infectious, then you would have no such choice; since you do have a choice it is clearly not infectious in this regard.
Let me end with an example. I have a tennis court on my property, and I allow you, my neighbor, to use it if you want, but I do not allow you to stop my other neighbor from using it. You cannot complain about not having this right, because the tennis court is my property and I decide how it may be used. You can, however, not allow any of your neighbors to use your swimming pool, and you are not obligated to do otherwise simply because of what I allow you to do on my tennis court. Giving you permission to use my property does not, in any way, transfer ownership of my property to you, or your property to me. Saying that this sort of agreement is "viral" or "infectious" is disingenuous, and completely ignores the bounds of the law.