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[bukimi]

I want to add something similar KazeeXXL mentioned:

I'm currently mailing with people with different parts of the world.
As we don't know each other, we keep e-mails text-only and "attached files" are only links to pictures and videos that were already published somewhere official (wikipedia, YouTube, etc.).

Yet, I happened to correspond with some people "just connected to Internet". They seemed to be unaware any AV is needed.
I started to get numerous spam from them, as my e-mail was on their contact list. I know they did not send any of it on purpose.
(fortunately I have separate e-mail for these purposes)
I wonder if any of their viruses got THEIR computer visibly affected in any negative way, but it's possible that it didn't.

Finally, I had to block e-mails from their e-mail address. Bad luck, isn't it?

Still, I bet their computer is, as already mentioned, spamming other people and attacking computers as an element of some botnet

EDIT PS. Times when computer virus was the one showing "Hello! You have a virus LOL" on your screen are gone.
Now the more invisible a virus is, the more efficiency it has.