The declaration that Guinea finally is free of Ebola virus transmission was good news for that nation. It also was a milestone. Each of the three West African countries ravaged by Ebola — the others were Liberia and Sierra Leone — now has broken the original chain of transmission that killed more than 11,000 people and sickened more than 28,000 in 10 countries since the epidemic was detected in March 2014.

That doesn’t mean there no longer is reason for concern. Flare-ups still are possible, and have occurred twice in Liberia since that country was declared Ebola-free in May. Continued vigilance is essential. Survivors must be studied carefully to determine why the virus reappears in some of them; testing suggests it can live on, for example, in the testes, and can re-emerge when someone’s resistance is lowered. There is some evidence transmission is possible via sexual contact or breast feeding. But some cases of re-emergence have no explanation thus far. Work must continue on experimental drugs and vaccines.

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It also is clear that the World Health Organization must be better prepared to deal with future outbreaks. The organization’s diagnosis of an international Ebola emergency came two months after Doctors Without Borders said the disease was spiraling out of control. The WHO must resist political demands from officials of stricken countries to minimize the severity of epidemics, and it must adopt some of the reforms proposed to ensure quick and effective responses in the future.

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This also is a good time to remember the heroism of responders who volunteered to go to West Africa to fight Ebola, and the misplaced hysteria that greeted their return in some parts of this country in particular. The world’s Ebola response, once it got underway, is a testament to what we can do when we all work together.