Viral licenses (was: wxWindows library...)

[amado.alves]

"...The GPL license is not viral..."
 
I sense there are two senses to this word "viral". I'm really interested in this so I'll appreciate any input. One sense is the GPL is viral because it spreads itself over derivatives i.e. forces derivatives to be distributed under GPL (if distributed at all, that is subsumed).* Is there another sense, perhaps more 'legal'? Thanks a lot.
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*Other open source licenses (not many) are viral in this sense.
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