When a Temporal Interventionist goes back in time to find out how a certain type of influenza broke out in several unrelated places at a time, he learns that yet another time traveler is spreading the disease to insure his time period does not develop as it did. Confusing? Only the way I tell it.
This is a great sci-fi short story which first appeared in Analog in December 2004. The narrative is well told from a first person point of view. I strongly recommend this as a quick read.