All licenses (except, in some jurisdictions, the public domain) have restrictions. They may be minor, they may be "common sense", but they are still restriction

All licenses (except, in some jurisdictions, the public domain) have restrictions. They may be minor, they may be "common sense", but they are still restrictions. The BSD license has restrictions on how code may be redistributed, in particular its requirements regarding attribution and non-endorsement. If I released some code with a license that said: This work may not be combined with any work which has a license containing more than twenty capitalized letters. Then that license would be incompatible with the BSD license, and it would be illegal to combine the two works. So if you define "viral" to mean "a license which can be incompatible with other licenses", then the GPL would fit that definition, but so would many other licenses (including the BSD license). Thus, it is not a useful definition. There is no useful definition of "viral" which covers the GPL but excludes the LGPL.