My training is as an epidemiologist (same as the folks at the CDC who do this kind of detective work). My observations and comments are thus:
Meningitis incubation periods can range from 2 days to 2 weeks depending on the causative organism. So, first the “cases” must be diagnosed. Each state has its own list of what are termed “reportable diseases.” On each state’s list are requirements for how quickly each of these diseases must be reported to the local health department (for meningitis, I did a quick look at a couple of states and it is usually by the end of the next business day after receiving lab results that confirm or suggest meningitis (again, there are some variations depending on the type of causative organism). Then the local health department needs to report the case(s) to the state health department.
So far we have to account for the time it takes to 1) incubate the disease, 2) diagnose a case, 3) get lab results, 4) get lab results to the local health department, and 5) get information from the local health department to the state health department.
This all has to happen for anyone reports anything to the CDC. The CDC’s list of reportable diseases is updated regularly and can be found here The lab results will trickle in from the states to the CDC. There are set thresholds for the anticipated number of cases for any disease and when this threshold is exceeded, then epidemiologists at the CDC will start their detective process. These thresholds range from immediate action for a single case of something like yersinia pestis (plague) or smallpox to the requirement of more cases for diseases with greater expected likelihood of happening. Keep in mind that state-level epidemiologists are often already starting their own investigation if they are seeing an unusual number of cases in their region.
This is what gets the process started. Then the detective work can take days to begin to tie together the loose threads to begin to see the pattern. The epidemiologists have questionnaires that they use to ensure that they are capturing the same information about each case so that these data can be compared. Once it has been determined that “all these people ate the potato salad at the picnic” or “all of these people received compounded steroid products through injections into the spine,” then, in the case of a tainted drug, the FDA would be called in to start looking at lot numbers and sources for the drug.
I think the CDC and the FDA did a terrific job in this latest situation. There are so many variables that have to be considered and, often, a great deal of time elapses before the CDC is even notified of the potential problem. They have teams that are ready to get on a plane at a moment’s notice to go track down potential disease causing agents and have established mechanisms for communicating potential problems to local healthcare practitioners to help ensure that every case is identified to the extent possible.