The likely suspect in the case of viruses is Satan. How do we know this, because the watchtower society gives us insight into such matters. Here is some wisdom from the Golden Age:
*Some people regard the common house-fly as comparatively harmless; but in point of fact it is one of the greatest disease carriers known.... One thing that tends to prove that the fly was originally created by the devil is that it loves every evil thing and hates every good thing. The odors that are most unpleasant to man,... are all pleasing to the fly....
THE mosquito has a bad record. He it is that carries the germs of malaria, yellow fever, [and other diseases].... If you have a disease of any of the above varieties, the mosquito loves to come and bite you and poison you,.... Then he goes off and bites some luckless chap and injects the germs of your disease into him. And if that does not show the disposition of the devil, will you please point out something that does?...
The Japanese beetle is said to be, at this time, our country's worst pest.... Perhaps the devil made this one, too; it looks like his work...
Probably the Lord made some of the useful insects. No doubt the devil made some of the others. The Lord is the better architect...
It’s not hard to extrapolate that the same devil that made the disease carriers also made the diseases themselves.
*C.J. Woodworth, "Forms of Insect Life," The Golden Age, Dec. 19, 1923, pp. 163, 164, 167, 168.
Freeman
(Satan’s little helper)