There aren't 20 active cases - there were 20 active cases at the beginning of September. There have been no active cases of Ebola in Nigeria since 22 September, but the incubation period for the disease is 21 days, and so they wait until 42 days after the last person gets sick (in case they infect an unknown person who then infects someone else) before declaring the country Ebola-free.
My understanding of the 42 day waiting period was slightly different. I didn't think it was 21*2 to allow for an intermediate unknown person. I thought it was that the 21 day incubation period is something like the 95th percentile, and the 42 day period is the 98-99th percentile.
From the article: "The 42-day period is twice the generally accepted maximum incubation period of the virus. However, some incubation periods are longer - that WHO said that in 95 percent of cases the incubation period was between one and 21 days. In 98 percent it was no longer than 42 days."
That's backed up by two citations, from the CDC and WHO. I think I'd also read something, before this epidemic started, that said the longest known incubation period was 21 days.
I've since found a bunch of articles, and one WHO press release, that say the 95th percentile was 21 days and 98th was 42 days. I'm a little skeptical of them, though: they all trace back to that one WHO press release, which is based on the West Africa epidemic, where (because transmission is endemic) it's quite possible that a patient was re-infected at a later date and that's why the incubation period seems longer. Unless I see evidence from a country where the cases are all contact-traced, I'd be inclined to believe the earlier studies about the virus and not the latest articles.
