FORT HOOD, Texas (AP) — U.S. troops preparing for deployment in Liberia to help fight Ebola are trading their rifles and body armor for rubber gloves, white germ-proof suits and gas masks.

About 500 soldiers were undergoing training Thursday at Fort Hood in Texas as they prepare to deploy to West Africa. Thousands of U.S. troops are already in the region helping fight the disease, which has killed thousands of people.

Some soldiers acknowledged apprehension about a mission focused on stopping a deadly disease rather than combat or counterterrorism. But they also expressed confidence that if any troops do fall ill, U.S. doctors can successfully treat them.

Meanwhile, six U.S. military planes arrived Thursday at the epicenter of the Ebola crisis, carrying more aid and American Marines into Liberia.

The fleet that landed outside the Liberian capital of Monrovia consisted of four MV-22 Ospreys and two KC-130s. The 100 additional Marines bring to just over 300 the total number of American troops in the country, said Maj. Gen. Darryl A. Williams, the commander leading the U.S. response.

A U.S. airman pauses to re-hydrate while setting up a 25-bed hospital near Monrovia, Liberia. | Getty Images

A U.S. airman pauses to re-hydrate while setting up a 25-bed hospital near Monrovia, Liberia. | Getty Images

The U.S. military is working to build medical centers in Liberia and may send up to 4,000 soldiers to help with the Ebola crisis. Medical workers and beds for Ebola patients are sorely lacking.

British Defense Secretary Michael Fallon said his country would provide more than 750 troops to help build treatment centers and an Ebola “training academy” in Sierra Leone. Army medics and helicopters will provide direct support. Britain will also contribute an aviation support ship.

British troops are expected to arrive next week in Sierra Leone, where they will join military engineers and planners who have been there for nearly a month helping to construct medical centers.

The German military, which has already been flying material such as protective clothing from Senegal to the worst-hit countries, planned to start a wider deployment of aid in mid-November. The military is expected to set up a clinic for 50 patients.

A U.S. airman connects electrical cables in the 25-bed hospital. | Getty Images

A U.S. airman connects electrical cables in the 25-bed hospital. | Getty Images