Such actions include reading directory entries, opening files, writing to files, etc. The system itself and other programs alike always perform these types of actions. There is a technique for detecting viruses that involves monitoring the system to watch out for such dubious actions. One example of such a dubious action would be opening a COM or EXE file in the write mode, while one would not expect executable files to be modified. Detection by behavior can also be used to detect and stop action relating to other aspects of the virus.
All viruses cause some change to the system in which they infect.
With normal DOS tools it is almost very easy to spot the change. It is of course very important that you have the initial state, file length, and respectively memory-size before infection, in order for you to detect any change. If you do not know if your system is virus free, you can detect any further virus spread when the virus changes the size of other files. File overwriting viruses are a group of simple viruses that just overwrite the targeted program with a copy of itself, thus destroying the original program. These viruses cannot be detected by their file size increases since they don not always increase it.