“Usually, an individual is not sick for three to five days after the onset of symptoms, which will fool you,” Dr. Ribner said. “You say, ‘Oh, you’re not going to be that sick.’ Then, around Day 5 to 7, they really crash. Their blood pressure goes down, they become stuporous to unresponsive, and they start to have renal and liver failure. This correlates with the enormous viral load, which is just attacking every organ in the body.”

A concern for health workers is that as patients grow sicker, the levels of virus in their blood rise and they become more and more contagious. The researchers at Emory tested patients and found high levels of the virus in their body fluids and even on their skin.At the peak of illness, an Ebola patient can have 10 billion viral particles in one-fifth of a teaspoon of blood. That compares with 50,000 to 100,000 particles in an untreated H.I.V. patient, and five million to 20 million in someone with untreated hepatitis C.

“That helped us to understand why, if this is only spread by body fluids, why it is more contagious than hepatitis A, B and C, and H.I.V.,” Dr. Ribner said. “It’s just that there’s so much more virus in the fluids they put out.”