If I may, a better phrased question is “what are the symptoms of a poorly designed virus“. The reason for the change is that a well crafted virus will not have any symptoms showing on your computer. There have been many examples of malware that have been in place and operating for years with nobody discovering them. This is because there are no symptoms to place anyone on alert.
Poorly designed malware will create almost any sort of symptom. This includes slowing performance, unusual behavior, and has been recently demonstrated causing hardware damage.
This of course does not include malware with the intention of causing problems. These include things like cryptolocker. However, the overwhelming majority of malware is not designed to cause problems but actually to harvest information.
