Very easy explanation.

If a virus infects a pc (windows) with e.g. Outlook on it, it sends an email containing itself (the virus) from this pc to every address in the outlook address book. It does this with every address it finds in the outlook address book as "sending address".

This means it will send (n=numbers of addresses in the address book) n*n mails containing itself. Maybe (n-1) * (n-1) if it does not send from the identical sender to the identical address.

I hope this explains why you often get mails from trusted pc-users simply by a different pc where the trusted user's address is on file.

Hope this explains it.

Greetings Thomas