The number of new cases in Liberia is “increasing exponentially,” according to a statement Monday by the World Health Organization (WHO), and “many thousands of new cases are expected in Liberia over the coming 3 weeks.”

There’s also a 20% chance that that the Ebola epidemic (as it is now called) will reach the U.S. by the end of September, according to experts writing in the journal PLOS Currents: Outbreaks (open access), Medical News Today (MNT) reported today, because Nigeria, where the outbreak has also spread, has many international travel links. An estimated 6,000 passengers fly from Nigeria to the U.S. every week.”

However, U.S. health care is expected to halt transmission, limiting outbreaks to isolated cases, according to the PLOS study, and “the risk of transmission of Ebola virus disease during air travel remains low,” WHO advised on August 14.

Meanwhile a fourth patient — a World Health Organization doctor — infected with Ebola virus has arrived from Sierra Leone in the U.S. for treatment, NBC News reported Tuesday, to be treated at Emory University Hospital (the third, Rick Sacra, M.D.., is being treated at the University of Nebraska Medical Center).

And an undisclosed number of people who’ve been exposed to the Ebola virus have been evacuated to the U.S. by Phoenix Air Group, an air ambulance company contracted by the State Department, Yahoo News reported Tuesday.

Meanwhile, a new model by Oxford University, published in the journal eLife Monday, “suggests that Ebola’s animal reservoir, fruit bats, could spread the disease in the animal kingdom and to humans through the dense forest that spans 22 countries,” the Washington Post reported Tuesday.

On the plus side, the Bill & Melinda Gates Foundation announced today (Wednesday Sept. 10) that it will commit $50 million to support the scale up of emergency efforts to contain the Ebola outbreak in West Africa and interrupt transmission of the virus.