Example: Virus, Spyware ,trojan horse
Virus is a program thats is sent to people. Which multiples according to the email adress contacts. Spyware is where a person secretly takes your information an example is a logger in a library computer. Trojan horse is where someone access your info from a link to access your contacts.
How to aviod
To avoid Malware always use common sense. If you can it is a good idea to setup a fire wall and use anti virus software.