What are computer viruses and how to deal with them?

These are chunks of code that are programmed to somehow change the way the computer they infect works.
These can be entire programs, or they can be small files with instructions for a computer.
These instructions can replace healthy files with the ones needed by the virus.
Typically, these instructions are contained in a .bat file.
It is enough to run such a file by double-clicking it, and the process of infecting the computer will start.
To combat them, there are programs called antivirus programs.