If someone downloads em, It will spread like a virus...
The only time, I remember, that a serious problem has occurred like this (where it effected you directly in-game instead of just hiding in your folders) was the Decal Virus.
This was in v0002. It was the default Decal file in a different filetype, and it appeared over your avatar ingame. If the host had it, it'd spread to any clients. If a client hosted, it'd spread to their clients.
But the mods introduced detections and deletion systems for this file, and killed it.
Luckily, this kind of pandemic isn't possible...decals can't do this...