The One Half DOS Virus (1994) was rather elegant. It would encrypt a user's hard drive over time until half was encrypted, while also decrypting files when they were needed. As long as the virus stayed on the machine things would run fine, but if it was removed the data was lost.
@elbienmaspreciado The only computer virus I managed to get back in the day (late 1980s) was the Ping-Pong virus.
It didn't really do much damage. You simply got a bounding ball on your screen now and then.
https://en.wikipedia.org/wiki/Ping-Pong_virus