To try to stop the virus’ transmission and restrain the outbreak, they need to identify every single person with whom patients have come into contact, isolate those at risk of harboring the illness, and monitor the entire network of people for symptoms.

That’s hard even if you know how each patient became infected. If you don’t, it can be downright herculean. The fact that the pathogen is passing through people without making them sick enough to seek care means that they may have unwittingly transmitted it for weeks before it came to the attention of authorities. Once a case is discovered, epidemiologists still need to figure out whom the patient might have exposed. But they also need to look in the other direction, too, to figure out by what unseen route the virus reached that person in the first place.

“The question is, what is their source of exposure?” said Laurence Burnsed, state epidemiologist for Oklahoma’s health department. “Who are we missing?”

Answering such questions requires state and county health agencies — overworked as they already are — to ramp up their efforts as fast as possible.