The notion that all viruses are bad is worth challenging. In 2008 paediatrician Dr Lawrence D. Rosen, MD published a blog post entitled Viruses and Health: Not all bad? which looked at two interesting cases and concluded:
In both of these cases, it appears that viral infections may indeed
confer some sort of protection or health benefit to the host. Is this
true in all circumstances? Of course not. But the converse - that
these viruses are always a health threat in all people, is equally
unlikely. It behooves us to be very, very careful when considering
eradicating viruses from our bodies, particularly when the cure is
potentially worse than the infection.
In addition, cancer research scientists are now looking at using (modified) viruses in order to combat this disease.ScienceDirect | Montana State University
Viruses are not mentioned in the Bible and whether they are technically living things is subject to continued discussion. In terms of their origin, the regressive and cellular hypotheses therefore don't entirely conflict with the Christian view; however the predominant view among creationists is that viruses are part of the original creation. Along with the rest of Creation, viruses were subject to The Fall, corrupting their original intended nature. The examples above show that viruses may be (or may have been) beneficial to health. In addition there is a theory called the Horizontal Gene Transfer (or horizontal gene flow) which may explain to some degree a purpose for viruses:
that genes picked up from somewhere in the environment rather than
inherited from parents may help to explain rapid adaptation and the
interesting pattern of DNA in animals.Answers in Genesis: Why Did God Make Viruses?