1. Computer viruses are programs that can copy themselves and infect computers without permission by attaching to other files or programs.
2. Viruses have existed since the early 1970s and have caused issues like slow performance, crashes, missing files, and unexpected pop-ups.
3. There are many types of viruses including file infectors, macro viruses, and polymorphic viruses that disguise their code, and each can impact computers in different ways such as corrupting files or redirecting web browsers.