Three homeless persons have died in Atlanta from tuberculosis as public health workers strive to contain an occurrence of the disease in four homeless shelters, state officials say.

In the outbreak linked to shelters in Fulton Country, 47 people have been infected, including two shelter volunteers, they reported.

Homeless persons are particularly vulnerable to tuberculosis, which is an airborne bacterium that can be contracted through close contact with an already infected person.

The transient nature of a city's homeless population increases their exposure, explained Nancy Nydam, a spokeswoman for the Georgia Department of Public Health.

"A homeless person may stay at one shelter one night, but go to another shelter the next night," she said.

The department sent a letter to area churches that often provide volunteer shelter workers recommending that such volunteers get screened to tuberculosis.

The letter included information on tuberculosis symptoms and how the disease is spread.

There is little risk to the general public since infection with tuberculosis requires prolonged sharing of an enclosed area and its air for at least eight hours, health officials said.

The most recent death was last week after the homeless patient was brought to the city's Grady Memorial Hospital.

Although tuberculosis in the United States has been on a gradual decline -- the 9,588 cases recorded in 2014 were a four percent drop from the year before -- there have been recent outbreaks in several cities including Atlanta, Los Angeles and Jacksonville, Florida.

In another southern state, Alabama, workers at a Mercedes-Benz auto plant in Vance were being screened after one employee tested positive for TB.

Health officials there said they were working with parent company Daimler AG to evaluate possible exposure.

"Only those people who were in close contact with the case need to be tested," they said in a statement.

Tuberculosis, caused by mycobacteria, usually attacks the lungs although it can affect other areas of the body.

Rates of infection vary globally; in many Asian and African countries as much as 80 percent of the population shows a positive result in tests to detect exposure, while in the United States that figure is only five to ten percent.

Most infections show no symptoms and the disease, remaining dormant, is termed latent tuberculosis.

Only about one in ten of such latest cases will become an active form of the disease.

If left untreated, about half of patients with active tuberculosis will die.