Loss, hunger and mass displacement aren't the only predictable elements of a natural disaster. There's also disease. With tens of thousands of hurricane survivors crowded into shelters around the country and others refusing or unable to leave spots contaminated by floodwaters, public health officials acted last week to try to control outbreaks of infectious disease -- and quell unfounded rumors about others.

While concerns remained about possible cases of West Nile virus, tuberculosis and leptospirosis, immediate efforts focused on treating and containing common gastrointestinal viruses, vaccinating children against measles and chickenpox, and getting survivors back onto drug regimens that Katrina disrupted. These, of course, were on top of other critical health concerns including dehydration, depression and a host of other medical conditions.

We talked with health officials and infectious disease experts about the biggest communicable disease concerns facing hurricane survivors.

* Is there any truth to reports of outbreaks of cholera and typhoid?

"It is extremely unlikely that cholera will be an issue. It just doesn't exist in that region," said Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases. "What has confused the picture is that there have been seven case reports, with at least four deaths, of . . . vibrio vulnificus," a cholera cousin that has caused pandemics in Africa and Asia. The four vibrio deaths were evacuees -- three in Mississippi and one in Texas. The disease-causing bacteria, found in seawaters around the Gulf and even in the Chesapeake Bay, pose little threat to healthy people. But the elderly and the immune-compromised are more at risk.

Health officials say they don't anticipate a large outbreak of vibrio infections, but have alerted first responders and directed them to treat it with the antibiotic tetracycline or Cipro.

Fauci called a typhoid outbreak "extremely unlikely."

* Are evacuees contagious?

If by this you mean, are they likely to spread disease to the community at large, officials say the answer is no.

"We're starting to hear reports that some of the folks evacuated could cause potential health risks to the cities they've gone into," said Von Roebuck, a spokesman for the Centers for Disease Control and Prevention (CDC), who dismissed the idea. Roebuck said health care workers are assessing all evacuees before they enter shelters and then vaccinating and separating them if necessary.

"Just because someone has been evacuated and they are living in a shelter doesn't mean they pose a health risk to others in the community. These folks shouldn't be stigmatized with that," said Roebuck.

Ultimately, however, disease control may be dependent on how well state and local authorities carry out CDC recommendations.

* What about people leaving shelters or just moving straight into communities?

The diseases health officials worry most about generally occur in crowded conditions with poor hygiene. These include diarrhea caused by bacteria such as E. coli, shigella and salmonella; viruses such as the norovirus (the bug that sometimes causes outbreaks on cruise ships, also referred to as the Norwalk-like virus) and rotavirus (the most common cause of acute diarrhea in children); and common upper respiratory infections.

With the dispersal of people into homes or smaller shelters, risk of widespread outbreaks plummets. Frequent hand-washing and good hygiene should limit cases of diarrheal illnesses in health care workers and families who take in evacuees.

"The best thing that can happen from a public health perspective is to get people out of the Astrodome," said Robert Edelman, professor of medicine and pediatrics at the University of Maryland School of Medicine and an expert in infectious diseases. "There's always potential for outbreak of disease when you have 10,000 people together."

* What types of diseases will likely affect evacuees?

The biggest immediate concerns are not exotic diseases but common diarrheal illnesses caused by drinking or coming into contact with contaminated floodwaters. Last week health officials closed a Biloxi, Miss., shelter with 400 people after some became ill with vomiting and diarrhea. Doctors suspected dysentery, an intestinal disease that can be caused by many organisms but is characterized by abdominal cramping and diarrhea.

Doctors in Houston reported Friday they had contained a viral outbreak that caused diarrhea and vomiting in some 700 people at the Astrodome and Reliant Arena, where 4,680 evacuees were still being housed; 40 were placed in isolation. The outbreak was blamed on the norovirus.

Other early reports of diarrheal illnesses were being investigated late last week by CDC, said the agency's director, Julie Gerberding. Because these illnesses can be easily transmitted by hand, health officials were urging frequent hand-washing in shelters. In the Astrodome, officials were urging people to use alcohol-based hand sanitizers, which is generally regarded as superior to hand-washing.

Respiratory illnesses, like diarrheal illnesses, pose a particular threat to small children, the elderly and the immune-compromised living in crowded conditions; these groups, said Gerberding, are at greater risk of dehydration or other complications. Risks are also exacerbated for people who have been living for a week or more without their usual medications.

Katrina's survivors are also considered at increased risk for hepatitis A (a liver inflammation spread by contaminated food or water or contact with an infected person) and tetanus (a neuromuscular disease that can follow a contaminated puncture injury). Fauci said the Department of Health and Human Services has shipped 90,000 doses of tetanus vaccine.

* Is vaccination part of the disease containment strategy?

Yes, a key part. The CDC is advising that adults and most children in shelters receive vaccines for influenza, varicella (chickenpox), MMR (measles, mumps, rubella) and hepatitis A unless they have a documented record of immunization.

The CDC is advising that all evacuees receive vaccinations against tetanus and influenza and that those over 65 or with a high-risk condition get a pneumonia vaccine. The agency also recommends that children 10 and under who are without medical records and are not living in shelters be "forward vaccinated" -- that is, considered up-to-date on immunizations. Children 11 to 18 should receive the following vaccinations: DTaP (against diptheria, tetanus, whooping cough), MCV (against meningitis) and flu.

Health care workers and emergency responders should get shots for tetanus and diphtheria, and those working directly with sick patients should get hepatitis B shots.

* What about tuberculosis?

Levels of tuberculosis -- a potentially fatal and transmissible bacterial illness -- are higher in inner cities, among minorities and among people with HIV virus -- "all three ingredients that would fit in this case," Fauci said.

Gerberding said some people in the shelters are known to have tuberculosis and were on medication for the disease, which keeps it from being contagious. There have also been investigations of potential new cases. However, prolonged contact with a highly infectious patient is needed for the disease to spread -- one reason health officials say this is not one of their most pressing concerns.

* Could children entering schools from shelters spread illnesses?

They are no more likely than any other children to spread illness, assuming their vaccinations are up-to-date.

The CDC has said if a child is well and was attending school before the hurricane, he or she should be allowed to attend school without further screening or quarantine. In Texas, some shelters are setting up vaccination centers; the state has issued a 30-day waiver on its requirement that children be fully vaccinated before they enter school.

Doug McBride, spokesman for the Texas Department of State Health Services, said displaced students enrolling in schools pose no increased health risk to Texas students. The CDC, meanwhile, is instructing health care workers to vaccinate both children and adults when in doubt, saying there is no known harm in repeated vaccinations.

"Immunization rates in some of these areas [that the children come from] are lower than they should be," said Trish Perl, an associate professor of medicine in the division of infectious disease at Johns Hopkins University School of Medicine. But officials say high immunization rates elsewhere around the country should protect children.

"It would be unusual for measles or rubella or the other childhood-vaccine-preventable diseases to emerge, but many of the people in these shelters are among the people in our country who already experience health disparities, and we're erring on the side of immunizing anyone for whom we have any reason to suspect their vaccination status is not up-to-date," Gerberding said in a press conference last week.

* How big a disease threat are the floating bodies we see in those horrific news images?

Corpses, while they make a disturbing sight, do not pose an infectious disease threat. "That's a myth," said Edelman. "No mosquito is going to bite a decaying body. Decaying bodies don't defecate."

Raw sewage poses a far greater threat. "It's the people who are going about their daily business who are contaminating the environment," said Thomas Fraser, an infectious disease physician at the Cleveland Clinic.

* I've also heard mention of diseases with icky names like trench foot and leptospirosis. What are those?

These are just two of the many reasons why officials want all residents to leave New Orleans and avoid continued exposure to contaminated floodwaters. Trench foot is a painful ailment that can occur when feet are immersed for a long period. Leptospirosis, initially marked by fever and aches, comes from exposure to infected animals or water that contains their urine. In worst cases, kidney or liver failure may result. The CDC has placed updates on these and other diseases on this Web page: http://www.bt.cdc.gov/disasters/hurricanes/infectiousdisease.asp.

* What about West Nile and other mosquito-borne illnesses?

"It's possible you could have a significant outbreak of West Nile," a virus that can be dangerous or fatal to the elderly and the immune-compromised, Fauci said. But we won't likely know for weeks. Edelman recalled that when Hurricane Camille barreled into the Eastern United States in 1969, heavy rains and the storm surge wiped out mosquitoes for about a month. Katrina is expected to have a similar effect.

Once the mosquitoes' breeding cycle is restored and eggs begin to hatch in standing water, residents in the Gulf area could be at greater risk for both West Nile and St. Louis encephalitis, another mosquito-borne illness. Widespread spraying of the area might curtail an outbreak. Whether toxic chemicals and oil in the water might hamper mosquito breeding is not known. Neither West Nile nor encephalitis is passed from person to person.

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Elizabeth Agnvall wrote recently for the Health section about women's heart health.

Vibrio vulnificus bacteria, as seen in an electron micrograph, have sickened and killed some evacuees.

Crowding evacuees into large shelters like this one, photographed last week in Gonzales, La., aids infectious disease transmission, say experts.