UPDATE — this extends our earlier coverage: DR Congo's Ebola Outbreak Reaches Sixth Province as Deaths Accelerate

DR Congo's Ebola outbreak has become the deadliest in the country's history, according to the BBC.

The outbreak was already the fastest-growing on record before crossing that threshold, the BBC reported. It is caused by the Bundibugyo strain, a rare species of Ebola virus.

Our earlier coverage traced the outbreak's spread to a sixth Congolese province as fatalities accelerated. DR Congo has fought more Ebola outbreaks than any other country.

Why it matters: DR Congo has largely contained past Ebola outbreaks within its borders, helped by experience from outbreaks caused by the more common Zaire strain. Bundibugyo has caused fewer recorded outbreaks globally, leaving responders with a thinner evidence base on its behavior. The record pace and record toll raise the odds of spread to neighboring states and increase pressure on the World Health Organization to consider declaring a Public Health Emergency of International Concern, the mechanism that mobilizes global resources and travel-related measures.

What to watch: whether case numbers keep climbing in newly affected provinces, and whether the outbreak crosses into a neighboring country.