Malware!
Viruses, Trojan Horses
Malware is software that has been developed for "Bad" uses. One example would be Trojan horses, that seems legit and makes you want to click the link or email, then proceeds to open a "Door" for someone who sent it to access your email or account.
Malware is usually disguised as something legit like a bank email, and by opening or clicking the link to it, you would allow viruses to copy itself and go to your contact spreading itself. If you want to avoid these, try using spam filters, don't open links or emails that come from someone you don't know or don't trust.