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In their search for the cause of a fatal outbreak in the Southwest, scientists hear echoes from other diseases that burst on the public consciousness in recent decades: Lyme disease, Legionnaires' and AIDS.

The newest illness is clearly different from the others. But it has come to light in a similar fashion. And experts believe the medical detective tools that eventually tracked down the cause of the other diseases will solve this puzzle, too."It usually starts with a cluster of cases in one geographic area, or a number of cases come to the attention of a single authority in some way," said Dr. Noel Rose of Johns Hopkins University. "Nobody begins to get excited until a number of cases come together."

In the Southwest, the first cases go back at least to March. But health experts didn't begin to notice a pattern until May. Now, 20 people have gotten sick and 13 have died, most of them with ties to the 17-million-acre Navajo reservation in New Mexico, Arizona and Utah.

Symptoms begin vaguely as flu-like fever, aches and congestion. Death can come within as little as a day. Like Lyme and Legionnaires' diseases, this one may have always been around but struck so irregularly that no one recognized it until now.

Perhaps the closest parallel to the new outbreak is Legionnaires' disease. It, too, is a lung disease.

Legionnaires' is a form of pneumonia that was first recognized when it struck people attending a American Legion convention in Philadelphia in 1976. In all, 221 people got sick and 34 died.

Scientists spent months tracking down the bacterium responsible. Now it is clear that the bug has long been around, and it strikes about 25,000 Americans annually.

In some ways, though, this cluster may be more challenging than Legionnaires'.

"That outbreak was focused in one hotel in Philadelphia," said Dr. Gary Simpson of the New Mexico Health Department. "This is an outbreak that is focused in an area that's larger than many states."

Experts say the discovery of Lyme disease shows how the tools of medical investigation can uncover the sources of emerging illnesses. In the mid-1970s, doctors noticed a cluster of children in a Connecticut town with arthritis.

The children seemed to get completely better between flare-ups.

Eventually, another curiosity of these cases stood out: The victims all had tick bites months before their arthritis.

With these clues, scientists eventually figured out that Lyme disease was caused by a form of bacteria harbored by ticks that lived on deer and mice. It turns out that the Lyme germ has long been a human pest.

Such diseases as Lyme disease and AIDS become apparent only when people change their habits. Experts believe AIDS was isolated in remote African villages until recent times, when people began moving into cities.

Lyme disease became a concern, at least in part, because people have moved from cities into suburbs where ticks and mice abound.

The Navajo outbreak "could be an old disease that has always been around and is just clustering," Rose said. "It could be a completely new disease. More likely it is a change in the ecology of the disease."