I know someone asked this five years ago, but I'm still confused. I found one in my game earlier while trying to make a story-like game. I had around three free models in my game, and had inserted an extra for a joke. (The joke was a model of a chat bubble, the others that I kept were a tree, a bench, and a real chat bubble script.) I suspect that the model of the chat bubble was what inserted the malicious script, but I'm not sure. The inFECtion model only appeared while in testing mode, (run/play mode,) and it included a script called "Script... Or Is It," which is suspicious all on it's own. After closing test mode, Roblox Studio crashed, and now I'm here. I'll be taking a break from Studio until I figure out what's going on. Once I do start developing again, I won't use free models as frequently.
From what I've seen, inFECtion scripts usually find all the models in the Workspace and clone themselves into them, making the script rampant in your game, or they may delete something in your game while testing. I've used free models before and have had this happen to me as well, but the damage done has never been permanent. Once you stop testing everything should go back to normal, and if you go into the model and delete the script, it should be fixed, although I'd suggest deleting the 'infected' model entirely, as you can probably find another 'non-infected' model. It sounds like in this case, something in this infected script overloaded your game making it crash, but it should be fine when you reopen studio; just delete the script. I think the word 'infection' in this case is really just meant to scare, and really isn't as bad as it seems. Only speaking from experience here though.
I wouldn't say to never use free models. I frequently use them as placeholders while I script something, and then later make my own. Just make sure to delete, or at least read any scripts that are in them!
dont use free models they can get access to your game and take your game so if you use free models check for that stuff and delete it
! I will try to explain everything as best as I can !
There are these things that people add in models that are called backdoor scripts, and backdoor scripts are typically an admin script which lets exploiters take advantage of server-sided effects. Backdoor scripts could also just be scripts allowing server-sided changes to take place via exploiters. The same applies to the free model “viruses.” And if you ever do insert a free model make sure to check the "children" of the free model, just in case if there is a script that may infect your game
You can check if there are any scripts like this:
Checking the children
Children are the children of something, for example; if there was a part with a script in it, the part will be the parent, and the script will be the child.
(click the little arrow thing to see if there are any children in there)
if there are more of the little arrows inside the parent click them
Searching script in workspace
If you press (Ctrl+Shift+X) or press search type Script and you will find scripts like that, and it's faster
Hope this helped lol