Different kinds of virus

A computer virus and email virus basically have the same modus operandi, the difference though is that an email virus would attach it self to an email message or automatically send itself using the addresses in the address book in order to infect the people who receive the email.
A computer virus is usually embedded in a larger program, often a legitimate piece of software. The virus will be run when the legitimate software is executed. The computer virus would load itself into the memory of the computer and then it will seek out any programs where it can likely attach itself. When a likely program is found then the virus would modify the file in order to add the virus’ code to the program. The virus would usually run before the actual legitimate program runs. Infection first before it commands the legitimate program to run. This process is so fast that no one would even notice that a virus was executed. With two programs now infected (the original program and the first infected program), the same process would be repeated whenever either program is launched worsening the level of infection.

After the infection phase, or even within the middle of the process of infection, the virus would usually start its attach on the system. The level of attach can range from silly actions like flashing messages on the screen to actually erasing sensitive data.