42 days
Ebola · 42 days to over
The World Health Organization rule for declaring an Ebola outbreak over: 42 days with no new confirmed or probable case, counted from the last possible exposure to the last case. WHO gives the reason itself: twice the maximum incubation period of 21 days.
When someone catches Ebola, they can feel fine for up to 21 days before they get sick. So how do you know an outbreak is really over? The World Health Organization waits twice that long. When 42 days pass with no new case, starting from the last person who could have caught it, the outbreak can be declared over. Nigeria reached that 42-day mark on 20 October 2014. Liberia reached it again on 9 June 2016.
What it supports
A real public-health rule with its number and its reason printed by the body that uses it. WHO's technical note (updated 4 March 2020) says the outbreak is interrupted "when no confirmed or probable Ebola virus disease (EVD) cases are detected for a period of 42 days (i. e. twice the maximum incubation period for Ebola infections)". The count starts the day after a recovered patient's second negative blood test, or the day after burial. WHO's fact sheet gives the incubation period as 2 to 21 days. Nigeria reached the mark on 20 October 2014. Liberia's 9 June 2016 announcement came 42 days after the last confirmed patient tested negative a second time. Red-cell storage is another 42-day line in medicine. Same family, different object.
What it does not
Not a law of the virus; 42 is a chosen safety margin (two incubation periods), not something Ebola does. Not proof the virus is gone: WHO says it can persist in some survivors for months, so watching continues for at least 6 months after. Not the rule for every disease; each disease's window depends on its own incubation period. Not a claim about who first set the rule or when. Not a 42-day human cycle. Not Adams.
ExactnessExact
Form42 as duration
Unitdays
Strengthattested
Hung7 November 2026