@LorenzoAncora >are called "viral" because their copyleft provisions require that any derivative works (either commercial or non-commercial) be licensed under the same terms
Such naming is an error.

You're allowed to develop derivative works for personal use, whether non-commerical or commercially in a business without being required to do anything (almost all proprietary licenses refuse to allow you to modify the software, but those aren't known as viruses?).

Combined works don't necessarily need to be licensed under the same license - you can choose a compatible license for your parts like MIT expat instead, although say the GPLv3-or-later would apply to the work as a whole (note that if you were to delete all GPLv3 parts and any copied lines from the MIT expat work, the license of such work is MIT expat).

Derivative works of any software are only allowed as per the license terms - most licenses totally forbid doing so (but those are not called viruses?), but the GPLv{2,3} does allow you to make derivative works - you're just not granted the power to make those derivative works proprietary.

>effectively "spreading" the license to all modified or combined code.
The GPLv{2,3} only applies to the parts that are licensed under such licenses - a proprietary license that tries to restrict the freedom that the GPLv{2,3} grants is not allowed (but the GPLv{2,3} is seen as the virus and not the proprietary license?).

The thing that actually does the spreading of the freedom is copyright holders making the choice to license under the GPLv2-or-later or GPLv3-or-later, not any virus action.