(Bloomberg) -- The Pentagon is working to determine how about 100 U.S. soldiers in Iraq and other parts of southwestern Asia got pneumonia, killing two so far, while saying that a biological toxin likely played no part in the outbreak.

``We've found no evidence of anthrax, smallpox or any other biological agent to which we can attribute the pneumonia,'' Colonel Robert DeFraites, of the office of the Army Surgeon General, told reporters in Washington.

Since March, soldiers deployed to the Iraqi conflict and the military operation in Afghanistan have contracted the infection of the lungs, and about 15 of those cases were serious enough for doctors to put the patients on respirators. One of the seriously ill soldiers died in June and another died last month, according to the Pentagon.

Ten of the 15 most serious cases occurred in Iraq, with others showing up as far away as Uzbekistan and Qatar, DeFraites said. The most recent case was confirmed July 30.

Concerns have arisen about the illness as the U.S. searches for chemical and biological arms that it says the Iraqi regime held before the war. While there are no known instances of such munitions being used against U.S. or British soldiers in the invasion, troops did find bio-chemical protection suits stored in Iraqi facilities as they moved through the country.

Teams Dispatched

The soldiers' deaths prompted the U.S. surgeon general to dispatch two epidemiological teams, one to Iraq and another to Germany, to study the patients and try to come up with recommendations on how to prevent the spread of the disease. The majority of the most seriously ill patients are being treated at a U.S. military base in Landstuhl, Germany.

Laboratory technicians are among the specialists in the team sent to Iraq.

The U.S. has been able to trace the cause of the infections in two of the most serious cases to a common bacteria, a condition called streptococcal pneumonia, Pentagon officials said.

``We are sparing no effort to further analyze and diagnose the nature of this condition,'' Dr. David Tornberg, deputy assistant secretary of Defense for clinical and program policy, said at the briefing.

DeFraites said that given the number of soldiers deployed in the affected areas -- around 150,000 in the Iraq operation alone - - the number of pneumonia cases ``is about the amount that one might expect'' and compares with the 400 to 500 cases of pneumonia the Army treats every year worldwide.

There have been 17 deaths from pneumonia in the armed forces in the past five years, DeFraites said.

The disease can be caused by exposure to certain bacteria, viruses, parasites and fungi, as well as environmental conditions such as dust, smoke or metals, DeFraites said.

The Pentagon recommended soldiers try to wear facemasks to limit their exposure to dust, and wet the ground when they need to sweep to avoid dust clouds. The Army is also reminding soldiers that smoking cigarettes lowers resistance to infection.