The recent Ebola outbreak in West Africa has claimed the lives of over 11,000 people to date, mainly in Liberia, Guinea, and Sierra Leone, as reported by BBC News. Today, it is a pleasure to report that these three countries at the very heart of the deadly epidemic have recorded their first week with no new cases since the outbreak began in March of last year.

While the number of new cases has fallen sharply since the start of 2015, the outbreak remains the worst epidemic of Ebola in human history: It has killed five times more people than all the other known Ebola outbreaks put together. A combination of ease of transmission, cultural practices such as the handling of the recently deceased by family members, ineffective medical infrastructure and difficulty in diagnosis has allowed the initial infection in a small Guinean village to kill thousands of people in at least six different countries.

The World Health Organization (WHO) has advised that we should remain cautious and vigilant: It only takes one undetected and untreated case for a new epidemic to spread. Looking at a report from the WHO (dated May 9th, 2015), it had already declared Liberia Ebola-free: 42 days had passed since the final confirmed case of the virus was buried on March 28th. The incubation time of the virus – the time between infection and the emergence of symptoms – is known to be 21 days, so a time period double that was chosen to absolutely confirm that the virus had been eradicated.

Two months later, as reported by USA Today, six cases of the viral infection resurfaced, including within the capital of Monrovia. Within hours, over 140 people had been exposed to the virus, and 16 were rapidly placed under quarantine. Although this flare up of infections was eventually curtailed, culminating in today’s announcement, it is clear that vigilance is still required for some time.

As the WHO report mentions, the hard work of thousands of medical professionals and volunteers, along with an international collaborative effort, is the reason that this announcement can be made today.

This devastating viral disease was traced back to a two-year-old boy playing near a tree that was home to insect-eating bats in his Guinean village of Meliandou. These common insectivorous bats also live peacefully in the roofs of nearby houses, which suggests that the reservoir for the Ebola virus could be hanging above the heads of these villagers as they sleep at night.

This, of course, raises a mystifying question: If the reservoir is so abundant, and so close to human populations, why haven’t there been far more outbreaks of the virus? Clearly, there is more to find out.