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Copyleft
Copyleft (also share-alike) is a concept of sharing something on the condition that others will share it under the same terms; this is practically always used by a subset of free (as in freedom) software to legally ensure this software and its modifications will always remain free. This kind of hacks copyright to de-facto remove copyright by its own power.
Copyleft has been by its mechanisms likened to a virus because once it is applied to certain software, it "infects" it and will force its conditions on any descendants of that software, i.e. it will spread itself (in this case the word virus does not bear a negative connotation, at least to some, they see it as a good virus).
For free/open-source software the alternative to copyleft is so called permissive licensing which (same as with copyleft) grants all the necessary freedom rights, but does NOT require modified versions to grant these rights. This allows free software being forked and developed into proprietary software and is what copyleft proponents criticize.
In the FOSS world there is a huge battle between the copyleft camp and permissive camp (LRS advocates permissive licenses with a preference for 100% public domain).