Below is a listing of some of the different properties a computer virus is capable of having and what the particular property is capable of doing. Keep in mind that not all viruses will have every one of these abilities.
Your computer can be infected even if files are just copied. Because some viruses are memory resident, as soon as a diskette or program is loaded into memory, the virus then attaches itself into memory and then is capable of infecting any file on the computer you have access to.
Can be Polymorphic. Some viruses have the capability of modifying their code, which means one virus could have various amounts of similar variants. This is also true with e-mail viruses that change the subject or body of the message to help from being detected.
Can be memory or non-memory resident. As mentioned earlier a virus is capable of being either memory resident where the virus first loads into memory and then infects a computer or non-memory resident where the virus code is only executed each time a file is opened.
Can be a stealth virus. Stealth viruses will first attach itself to files on the computer and then attack the computer; this causes the virus to spread more rapidly.
Viruses can carry other viruses. Because viruses are only software programs a virus may also carry other viruses making the virus more lethal and help the primary virus hide or assist the primary virus with infecting a particular section of the computer.
Can make the system never show outward signs. Some viruses can hide changes made, such as when a file was last modified making the virus more difficult to detect.
Can stay on the computer even if the computer is formatted. Some Viruses have the capability of infecting different portions of the computer such as the CMOS battery or master boot record. Finally, if a computer is completely erased and the virus is on a backup disk it can easily re-infect the computer.