One year ago, the World Health Organization declared an outbreak of the Ebola virus in the Democratic Republic of Congo after the deadly disease was discovered in the country’s North Kivu Province.

In the months since, the virus has continued to spread — two weeks ago, the outbreak was declared a global health emergency — with more than 2,600 people infected and over 1,800 killed, according to W.H.O., making it the second-deadliest Ebola outbreak in history.

On Thursday, Congolese health officials confirmed four cases of Ebola and the first active transmissions of the disease after two more people linked to a man who had died of the disease become infected in Goma. The city of more than two million people is a major transit and trading hub that thousands of people pass through each day. The city borders Rwanda, which temporarily shut the border crossing there on Thursday as fears of the disease heightened.

Experts warn that the disease’s spread could get worse. How did the outbreak get to this point, where does it stand now and what comes next? Here’s what you need to know.