To oversimplify this greatly, it isn’t what the virus is, it is what it attacks.
If the virus is killing off say a red blood cell, the white blood cells can attack it. But what if the virus is attacking the white blood cells. First they must fend off the attack then regroup to kill the virus.
Also you viruses like HIV which mutate at the drop of a hat. If the body fights off a virus that looks like the letter “C” it will again and again defend itself from any virus that looks like the letter “C” but what if in the process the virus that looks like the letter “C” mutates so that now it looks like the letter “C” but also “D” and “E.”
Now you can see where it gets complex. And my explination is an oversimplification.