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Viral license is a copyright license that allows derivative works only when licensed under a compatible license to the original. Licenses that require derivative works to be licensed only under compatible licenses include copyleft licenses which include several common open source and free content licenses, such as the GNU General Public License (GPL) and the Creative Commons Attribution-ShareAlike license.
The term is most often used to describe the GPL, which requires that any derivative work also be licensed with the GPL. The viral component is described as such because the licenses spreads a continuing use of the licenses in its derivatives. This can lead to problems when software is derived from two or more sources having incompatible viral licenses in which the derivative work could not be re-licensed at all.
Although the concept 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.
As an example of viral licensing outside software, after it was revealed that French author Michel Houellebecq plagiarized sections of Wikipedia articles in his novel La Carte et Le Territoire, some commentators said that this automatically made his entire book licensed under the ShareAlike license.
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. 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, Steve Ballmer 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".
Criticism of the term 
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." Richard Stallman has described this view with an analogy, saying, "The GPL's domain does not spread by proximity or contact, only by deliberate inclusion of GPL-covered code in your program. It spreads like a spider plant, not like a virus."
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.
The interoperability clauses are often pragmatically inoperative due to the vigorous enforcement and strict interpretation of the GPL as it related to integration, aggregation, and linking. It is argued that most forms of incorporation, aggregation, or connectivity with GPL-licensed code is a derivative work that must be licensed under the GPL. In recent years, a number of communities using GPL incompatible licenses have dropped efforts and support for interoperability with GPL-licensed products in response to this trend. Some developers and communities have switched to the GPL or a GPL compatible license in response, which critics and supporters alike agree is intentional end result. A number of prominent free software advocates, such as Linus Torvalds, have vocally opposed this restrictive interpretation and enforcement.
See also 
- Golden, Bernard (2005). "3". Succeeding with Open Source. Addison-Wesley. p. 44. ISBN 978-0-321-26853-2.
- Meeker, Heather J. (2008). "2". The Open Source Alternative. John Wiley and Sons. p. 11. ISBN 978-0-470-19495-9.
- Vixie, Paul (2006-03-06). "Re: Section 5.2 (IPR encumberance) in TAK rollover requirement draft". IETF Namedroppers mailing list. Archived from the original on 2007-09-27. Retrieved 2007-04-29.
- "General Public Virus". Jargon File 2.2.1. 1990-12-15. Retrieved 2007-04-29.
- Hackvän, Stig (September 1999). "Reverse-engineering the GNU Public Virus — Is copyleft too much of a good thing?". Linux Journal. Retrieved 2007-04-29.
- Stewart, Bill (1998-10-08). "Re: propose: `cypherpunks license' (Re: Wanted: Twofish source code)". Cypherpunks mailing list. Retrieved 2007-04-29.
- Buck, Joe (2000-10-10). "Re: Using of parse tree externally". GCC mailing list. Retrieved 2007-04-29.
- Griffis, L. Adrian (2000-07-15). "The GNU Public Virus". Retrieved 2007-04-29.
- Mundie, Craig (2001-05-03). "Speech Transcript - Craig Mundie". New York University Stern School of Business. Retrieved 2008-08-23.
- Newbart, Dave (2001-06-01). "Microsoft CEO takes launch break with the Sun-Times". Chicago Sun-Times. Archived from the original on 2001-06-15. (Internet archive link)[dead link]
- Byfield, Bruce (2006-08-29). "IT Manager's Journal: 10 Common Misunderstandings About the GPL". Retrieved 2008-08-23.
- David McGowan (2005), "Legal Aspects of Free and Open Source Software", in Joseph Feller, Brian Fitzgerald, Scott A. Hissam, Karim R. Lakahani, Perspectives on Free and Open Source Software, MIT Press, p. 382, ISBN 0-262-06246-1
- Poynder, Richard (2006-03-21). "The Basement Interviews: Freeing the Code". Retrieved 2010-02-05.
- "Frequently Asked Questions about the GNU Licenses". Free Software Foundation. 2008-06-24. Retrieved 2008-08-23.
- Raymond, Eric Steven (2002-11-09). "Licensing HOWTO". Retrieved 2010-03-21.