A viral license is a term used to describe a license that allows derivative works only when licensed identically to the original. Such licenses exhibit a viral phenomenon as they spread from work to work. The term is most often used to describe the GNU General Public License (GPL). It is generally used to distinguish copyleft licenses, such as the GPL, from more permissive licenses, such as the BSD license, which allow more choices for licensing derivative works.
The GPL requires that any derivative work be licensed with the GPL. This applies even if only a small amount of GPL code is used in the derivative work. Thus the GPL is referred to as a viral license because the original GPL product "infects" the derived work by forcing it to also be licensed under the GPL. This intention is not denied by the GPL authors who have written the license with exactly this goal and are sure that propagation of the GPL has a positive effect. However, this can lead to problems when software is derived from two or more sources having incompatible viral license. Thus the derivative work cannot be re-licensed at all.
Since the term "viral" has mostly negative associations and is considered by some to be pejorative, "reciprocal" has been suggested as a better term.
Although the term viral license is generally associated with licenses that promote free content, many proprietary licenses also have viral characteristics. For example, original equipment manufacturer source code software distribution agreements generally grant licensees the right to redistribute copies of the software, but restrict what terms can be in the end user license agreement. However, derivative work is much less common with proprietary licensed work and so the viral phenomenon is not as evident.
The term 'General Public Virus', or 'GNU Public Virus' (GPV), has a long history on the Internet, dating back to shortly after the GPL was first conceived. Many BSD License advocates used the term derisively in regards to the GPL's tendency to absorb BSD licensed code without allowing the original BSD work to benefit from it, while at the same time promoting itself as "freer" than other licenses. Microsoft vice-president Craig Mundie remarked "This viral aspect of the GPL poses a threat to the intellectual property of any organization making use of it." In another context, Bill Gates declared that code released under GPL is useless to the commercial sector (since it can only be used if the resulting surrounding code becomes GPL), describing it thus as "a cancer that attaches itself in an intellectual property sense to everything it touches".
According to Free Software Foundation compliance engineer David Turner, the term viral license creates a misunderstanding and a fear of using copylefted free software. David McGowan has written that there is no reason to believe the GPL could force proprietary software to become free software, but could "try to enjoin the firm from distributing commercially a program that combined with the GPL'd code to form a derivative work, and to recover damages for infringement." If the firm "actually copied code from a GPL'd program, such a suit would be a perfectly ordinary assertion of copyright, which most private firms would defend if the shoe were on the other foot."
Popular copyleft licenses, such as the GPL, have a clause allowing components to interact with non-copyleft components as long as the communication is abstract, such as executing a command-line tool with a set of switches or interacting with a Web server. As a consequence, even if one module of an otherwise non-copyleft product is placed under the GPL, it may still be legal for other components to communicate with it normally. This allowed communication may or may not include reusing libraries or routines via dynamic linking — some commentators say it does, the FSF asserts it does not and explicitly adds an exception allowing it in the license for the GNU Classpath re-implementation of the Java library.