That's an incredibly high rate of incidence, because there were only about 200-250 Marines who went ashore at all, reports CBS News National Security Correspondent David Martin.
Two of the Marines were flown from the USS Iwo Jima warship off the coast of Liberia to a U.S. medical center in Germany on Saturday and 31 others were flown from the ship Sunday to the Bethesda Naval Medical Center in Maryland, reports Martin.
"All the Marines that were involved with the QRF [Quick Reaction Force], they're testing them and if they need to be evacuated then they will be," Master Sgt. John Tomassi, spokesman for the U.S. European Command in Stuttgart, told CBS Radio News.
The Marines, members of the 26th Marine Expeditionary Unit based at Camp Lejeune, N.C., were in Liberia in mid-August as part of a U.S. quick-reaction force of about 150 U.S. troops. They operated from an airport outside Monrovia, the capital.
U.S. troops normally receive an anti-malarial drug regimen before deploying to a country like Liberia where there is risk of getting the disease.
Details on the sick Marines' condition was not immediately available.
Malaria is transmitted by mosquitoes that breed in stagnant water and tall grass.
The disease kills 3,000 children a day in Africa and robs the continent of millions of dollars in lost productivity, the United Nations said in a report early this year.
The mosquito-borne disease infects 300 million people a year in the poorest continent and has become increasingly resistant to drugs, said the World Health Organization and the United Nations Children's Fund, UNICEF. Yet there are ways to control the disease, they said.