File infector viruses fix on executable files that have the .com and .exe extensions. When you open any of these infected files, you unintentionally launch the virus. The corrupt code then https://www.pat.org.uk/technology/04/2021/what-are-the-common-types-of-computer-viruses/5870/  takes over the program you wanted to launch and spreads to other executable files you have on your hard drive or network.

It costs a lot of money, honestly I do not understand how people can save up such a sum, in my job it certainly will not work.