Some viruses have the capability of modifying their code which means one virus could have various amounts of similar variants. Depending on the virus can be memory resident virus which first attaches itself into memory and then infects the computer. The virus can also be Non memory resident which means a program must be ran in order to infect the computer. 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 and infect that system and also infect with the other virus as well.
Because viruses are generally written by different individuals and do not infect the same locations of memory and or files this could mean multiple viruses can be stored in one file, diskette or computer. Can make the system never show outward signs. Some viruses will hide changes made such as when infecting a file the file will stay the same size. Can stay on the computer even if the computer is formatted. Viruses have the capability of infecting any file however will generally infect executable files or data files such as word or excel documents which are open frequently.