According to CDC director Thomas Frieden, the maximum incubation period for the Ebola virus (that is, the time interval from infection with the virus to onset of symptoms) is 21 days. That is the length of period CDC is monitoring people who are at risk of contracting the Ebola virus.

Even Nina Pham’s dog is being monitored for 21 days:

But what if Ebola patients can remain asymptomatic for more than 21 days? It’s not the norm, but it does happen, according to a statement released yesterday by the World Health Organization:

Recent studies conducted in West Africa have demonstrated that 95% of confirmed cases have an incubation period in the range of 1 to 21 days; 98% have an incubation period that falls within the 1 to 42 day interval.

We sure would like to hear more about those “recent studies.”