clamav wasn't installed as we don't currently receive virii by mail,
and new virii mutates too much to have an up-to-date signature database.

But https://www.sysnove.fr/blog/2016/07/comment-lutter-contre-virus.html
(found by @ledesillusionniste) documents the existence of several others
databases, some up-to-date.

We can give it a try.