Today's term is about virus. Not talking about the Covid-19, but about computer virus.
Computer virus shares the "virus" name, because they share common characteristics. They replicate themselves and spread from host to host.
A computer virus, is a self-replicating program, that produces its own copy by modifying other computer program, computer boot sector or document. It is generally transmitted via file downloads, infected disk/flash drives, email attachments, or within the infected network.
To countermeasure it as in common sense, we download files only from trusted source, use external disk/flash drives only from trusted source, do not open email or click link or attachment from unknown sender, install and keep anti-virus up-to-date.