A software virus could consume a great quantity of system resources, blocking the normal operation and gaining control of different functions that block the user. And that, in a first look, is precisely what an antivirus software might do in common conditions! Each day some individuals with too much free time try to develop code for messing up others' computers and that is equal to more signatures in the search and compare algorithm for each single archive. Most antivirus software works in that manner; it has a little virus portion and uses it to check if some archive has the same signature.
It's relevant to know that precisely for this reason, an antivirus could be very inefficient when there's no available signature to check for. In the other side, they need use a great amount of electricity to scan archives and make the computer slow. Each active program at startup also adds more time and especially an antivirus because it normally takes even control of the kernel or other important system processes.
All of it is thinking in security although it's not the first occasion in which some protection finishes creating more damage than without it. Think for instance in a hard case of silicone designed for smartphones, very tight. When some hard tiny objects fall inside, between the case and the smartphone, with movement it creates scratches. Was then the device protected? It was the cause!
Following some simple rules (basically never execute code coming from a non trusted source), it's very difficult to get bothered by some program with noxious code thus some people never use antivirus software. Also, any firewall that gives information about what's happening and asks for permissions before doing sensitive activities could be better than any antivirus.