They reproduce by dividing themselves in half, meaning 100% of the DNA comes from one parent. In a mammal, the offspring gets half the DNA from mom and half from dad. If dad has a disease and mom doesnt, there is a 50% chance the offspring won't get the disease. In a virus, if the virus has a disease, there is a 100% chance the offspring will get it because the offspring is basically just a clone of the parent, called asexual reproduction. So 100% of mutations are passed from viral parent to viral offspring, while 50% is the maximum chance a mutation has of being passed to a sexually reproduced offspring.

For example, if a virus is resistant to a medication, all of its offspring will definetly be resistant too. If a human is allergic to a medication, there is, at maximum, a 50% chance they will pass that allergy on to their offspring. Also, a virus multiples way more times in its life than an animal. A virus can accomplish in a year what takes multicellular organisms decades to accomplish, simply because it divides so many more times.