The full health consequences of Hurricane Katrina probably never will be known.
The frantic effort to rescue and treat thousands of stranded survivors left little in the way of records describing their illnesses and injuries. The long-term health effects of the storm and flooding on the scattered evacuees will be chronicled in a hundred places and will be hard to document and analyze.
At best, epidemiologists may be able to sketch a picture of who suffers from what from now on, as a major American city is brought back from a near-death experience.
Teams from the Centers for Disease Control and Prevention fanned out across this damaged city last week, trying to find out what information was available and laying the groundwork to capture as much more as possible.
Despite the gaps in the record, what seems almost certain is that this city's epochal disaster will not be followed by plague and pestilence.
"We do not generally see massive outbreaks after these events," said Ali Khan, the physician and epidemiologist who is the CDC's team leader in Orleans Parish.
This is true even of natural disasters in the developing world. Khan helped head up the World Health Organization's response to last winter's tsunami in Indonesia. The populations of dozens of villages and towns were instantly reduced to primitive living conditions, living in hand-built shelters, defecating on the ground and drawing water from streams.
"That was hundreds of thousands of deaths and damage over multiple countries," he said. "We had a handful of cases here and there of measles, a handful of cases of dysentery that were quickly taken care of. So there was not these large outbreaks that people fear."
There are many reasons for this, the most important being the robustness of the human body and its immune system. Although the floodwater covering much of New Orleans is highly polluted, few if any people are drinking it, and mere contact is not enough to cause disease, except in rare cases.
One of the ailments floodwater might cause is infection with Vibrio vulnificus, a bacterium that resides in warm coastal waters. It can infect wounds or cause illness in people who eat raw seafood. People with weak immune systems or alcoholics are the most vulnerable. Wound infections with V. vulnificus are fatal 20 percent of the time; food-borne cases more often.
So far, the CDC has heard of fewer than a dozen cases in the hurricane zone, with several fatalities. Late last week, Thomas A. Clark, 35, a medical epidemiologist from the federal public health agency, was looking for more.
"Have you had any cases of Vibrio vulnificus?" he asked Wanda Eppling, a supervisor in the microbiology lab at Ochsner Foundation Hospital on the northern border of New Orleans.
"No, we haven't seen any, but I hear East Jefferson had one," she said, referring to a nearby hospital.
"I talked to them and they said it was actually Vibrio alginolyticus. That's something different, although it's probably salt water-related, too," Clark said.
Ochsner Hospital survived the storm and was not flooded. Lab technicians had to aim fans on their microbiology and chemistry analyzers to keep them cool enough to operate during the days the hospital was running on auxiliary power. For Clark's records, Eppling called up the hospital's computerized database and printed out the blood culture results since Aug. 28 -- the day before Katrina struck.
Dressed in a plaid short-sleeved shirt, cargo shorts and three days of beard, Clark was on his third stop of the day. He was part of a CDC effort to put in place a mechanism to capture the current and future health status of the Katrina victims who didn't evacuate.
The agency hopes the area's working hospitals -- fewer than a half-dozen out of nearly 20 -- will use a one-page questionnaire to gather basic information on everyone who comes to the emergency room or is admitted.
"The trick is to come up with some data that are scientifically useful that other people can read and say, 'If this happens here, this is what we can anticipate,' " he said.
CDC epidemiologists will collect the data from the hospitals, analyze it once a week, and provide reports to the state and city health departments and the hospitals themselves. They will also go back and extract information from ER records to learn the pattern of illness and injury the event wrought, at least in those who made it to hospitals that were still functioning.
A study of similar data on the effects of flooding in Missouri in 1993, which displaced 60,000 people, found the flood-related problems equally split between injuries -- most commonly sprains and lacerations -- and illnesses, primarily intestinal and skin problems. After Hurricane Floyd in 1999, a survey of 20 North Carolina hospitals found that skin problems, dog bites and the interruption of basic medical needs (such as dialysis and supplementary oxygen) were the immediate problems, followed by insect bites, diarrhea and asthma triggered by mold.
Neither study turned up any outbreak of contagious disease.
Katrina may be somewhat different. Norovirus, which causes a nonfatal intestinal illness, has been diagnosed in numerous evacuees in the Houston Astrodome, officials said.
What is almost certainly lost to scientific analysis is information about the spectrum of illnesses treated by emergency medical teams in the giant staging areas here and in Baton Rouge as large numbers of the city's poorest, sickest and least mobile citizens were removed en masse.
"We were lucky to generate a chart on them of any type. We were lucky to record a name. The paper work from the first few days was poor at best," said Erik Larsen, 54, an emergency medicine physician from White Plains, N.Y., who helped lead the field hospital at Louis Armstrong International Airport.
Larsen's comment came as the CDC's Clark, accompanied by two doctors from the agency's Epidemic Intelligence Service, visited the airport triage site to see what information they could glean there.
Activity at the airport was winding down rapidly. On successive days late last week, the field hospital treated 182, 158, 148 and 219 patients. At the height of the evacuation, it cared for more than 2,000 people over two days.
Larsen said the staff witnessed what happens when modern treatment of chronic disease is suddenly interrupted and underlying illness is worsened by stress and environmental conditions. There were lots of people with out-of-control blood pressure, diabetes or seizures, and people with innumerable less common problems such as mental illness, multiple sclerosis and Parkinson's disease whose conditions got worse.
"We saw people seizing who hadn't taken their anticonvulsant medicines, we saw people who were alcoholics seizing because they didn't have any alcohol, and we saw people seizing because of total electrolyte imbalance from nothing to drink, nothing to eat and being out in the hot sun," Larsen said.
The long-term consequences of this, however, are likely to be minimal. The interruption of medical therapy was brief, and in many cases the evacuees' physiological state differed little from that experienced by countless Americans every day.
For example, 36 million people in the United States have hypertension (high blood pressure), and the disease is well-controlled in only 40 percent. About 18.2 million people have diabetes, but 5.2 million are unaware of it. Numerous studies have shown that people infected with the AIDS virus can safely stop their drugs for a week or so with no bad results.
The social and psychological consequences are another issue.
Sitting at a small table over a stained carpet on a concourse at the airport, Larsen tried to describe the days immediately after the storm and flood.
"The sadness of some of the stuff we have seen here is overwhelming," he said. "You see these people with little children, barefoot and holding all the things they own folded over in a sheet, people who have no idea where they're going or what their future is . . . "
His voiced choked and his eyes momentarily clouded with tears. Then he composed himself and continued.