The first step in combating any infectious disease outbreak is detection. Without widespread testing, health officials have little sense of who is infected, when to treat patients and how to monitor their close contacts.

In that sense, the bird flu outbreak plaguing the nation’s dairy farms is spreading virtually unobserved.

As of Monday, the virus had infected 157 herds in 13 states. But while officials have tested thousands of cows and are monitoring hundreds of farmworkers, only about 60 people have been tested for bird flu.