What parts of a computer do viruses infect?

Their are many parts of a computer that viruses infect. Most viruses infect a certain file or program so that whenever that file is run, the virus is also executed.

Some viruses called Master Boot Record viruses infect the Master Boot Record of your computer. The Master Boot Record is the partition of your computer first executed when you boot up your computer. When you turn on your computer, it runs all the programs inside the Master Boot Record partition and then passes control to other child processes. Viruses in this class infect the Master Boot Record so that whenever your turn on your computer, the viruses runs at startup too. These viruses are very tricky to find as they hide in a very deep part of your Operating System and manually deleting them could damage your computer.