Star Surgeon by Alan Edward Nourse

It was a virus, beyond doubt. The electron microscope told them that, now that they had the substance isolated and could examine it. In the culture tubes in the _Lancet_'s incubators, it would begin to grow nicely, and then falter and die, but when guinea pigs were inoculated in the ship's laboratory, the substance proved its virulence. The animals injected with tiny bits of the substance grew sick within hours and very quickly died.

The call to the Hospital Ship was canceled as the three doctors worked in feverish excitement. Here at last was something they could grapple with, something so common among the races of the galaxy that the doctors felt certain that they could cope with it. Very few, if any, higher life forms existed that did not have some sort of submicroscopic parasite afflicting them. Bacterial infection was a threat on every inhabited world, and the viruses -- the tiniest of all submicroscopic organisms -- were the most difficult and dangerous of them all.