Exactly how a virus does this depends on its type, but can include propagation over removable media such as USB drives, networks, or network-based activities such as user downloads.
There are also dedicated hardware appliances that act only as firewalls, and there is software that can be installed on PCs to act as a firewall as well.
This is the memory that is lost when the computer is powered off, and the memory that is reloaded when the computer boots.
Many people use the term memory to refer to a computer’s hard disk drive (HDD) or persistent storage – the storage that is not lost when the computer is turned off.
A virus is a computer program written by someone, with the presumed intent of spreading and causing grief.
Like a human virus, a virus makes the infected computer “sick”: it causes poor performance, crashes, lost files and data, or more.