Consider some hardware problems. Software Threats Viruses are not the only things that can cause damage. Consider some software problems. Summary A virus is a program that reproduces its own code. Viruses balance infection versus detection possibility.
Some viruses use a variety of techniques to hide themselves.
On some defined trigger, some viruses will then activate. Viruses need time to establish a beachhead, so even if they activate they often will wait before doing so. Not all viruses activate, but all viruses steal system resources and often have bugs that might do destructive things. The categories of viruses are many and diverse. There have been many made and if you get one it should be taken seriously.