How do viruses spread?

There is not one way in which virus spreads. In thousands of ways, viruses can spread from one computer to another or from one network to another. The most common ways through which viruses can spread are:
· Booting of system with an infected media
· Opening a file infected with virus
· Downloading from unreliable source
· Opening unknown email attachments
· Executing an infected file
· Using floppy disk or any other media that are exchangeable
· Using shareware(software that is available free of charge and often distributed informally for evaluation)
· Using pirated software
· Extensive use of USB's without security system
Through other ways also, viruses can transfer or spread but these are the common ways in which it spreads.