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In the mid-1900s, health experts said the U.S. had won the war on infectious diseases, thanks in part to vaccination and stepped-up sanitation.
Don't relax just yet. Climate changes, uneven vaccination, globe-trotting travelers, and other factors have contributed to a resurgence of some types of bacteria, viruses, and disease-carrying insects.
Here are 10 U.S. states that have experienced outbreaks of a rare or exotic disease. Some, like West Nile, are relative newcomers; others, like dengue fever, are old scourges making a comeback.
Few states have escaped West Nile, a mosquito-borne virus that has infected more than 30,000 people and killed countless crows and other birds since entering the U.S. in 1999.
But lately Arizona is catching the worst of it. In 2010, it had 107 cases of an especially virulent form of the disease that can cause seizures, nerve damage, and even death.
Although the virus spreads to the brain in less than 1% of cases, people over 50 are at the highest risk. Arizona, a favorite retirement destination, is reminding residents to wear insect repellent and eliminate standing water in a "Fight the Bite" campaign.
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Every few years, whooping cough (or pertussis) resurfaces in the U.S. In 2010, California reported 9,477 cases of this highly contagious, potentially deadly bacterial infection, the largest outbreak since the 1940s. (Health officials say a falloff in vaccines, including the Tdap booster shot for teens and adults, is to blame.)
As if that weren't enough, California also has a disproportionately high number of cases of typhoid fever, an infection spread through contaminated food and water that causes stomach pain, weakness, and a sky-high fever. In 2009, California accounted for 90 of the 400-odd cases in the U.S.
In the early 1990s, an unusual respiratory disease struck dozens of healthy adults in Colorado, New Mexico, Arizona, and Utah, killing half of them. The "Four Corners outbreak" was eventually pinned on hantavirus, which spreads via mouse waste. (Humans can be exposed by drinking from dirty cans or inhaling dust in rodent-infested buildings.)
Although the virus was lurking in the U.S. for decades, unusually heavy rainfall in the 1990s is blamed for driving up rodent, and thus virus, numbers. Since 1993, Colorado has had 75 of the country's 568 cases of hantavirus pulmonary syndrome, a deadly respiratory complication.
Although it takes its name from the bucolic Connecticut town where a mysterious outbreak of arthritis-like symptoms was first described in 1975, Lyme disease is now most common in Delaware. The First State reported 111 cases for every 100,000 inhabitants in 2009—a rate 42% higher than in Connecticut.
Clusters of Lyme cases can also be found as far south as Maryland and as far west as Minnesota. The ticks that shuttle the Lyme disease-causing bacteria between deer, mice, and men have a wide foothold—and thanks to warming winters that fuel the tick population, it could be getting wider.
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The Sunshine State's heat and humidity create an ideal breeding ground for mosquitoes—and the diseases they carry, like malaria and dengue fever. While insect control efforts in the mid-1900s helped stamp out these diseases, dengue virus could be making a comeback.
The epicenter is Key West. In 2010, 66 people there were infected with dengue, known as breakbone fever for the excruciating joint pain it causes. As with Lyme-carrying ticks, many experts fear that the warming climate in the U.S. will allow dengue-carrying mosquitoes to spread.
Nearly all cases of malaria in the U.S. are imported from abroad, but a related disease called babesiosis ("bab-EE-see-OH-sis") has been indigenous to the coastal northeast for decades.
The first person known to have contracted the Babesia parasite was bitten by a rodent tick in 1968 while vacationing on Nantucket, off the coast of Massachusetts. The disease remains prevalent on the island, where 60% of rodents are estimated to carry Babesia and 16 of the state's 78 cases in 2009 occurred.
The disease damages red blood cells and can lead to severe anemia, especially in the elderly or people with weak immune systems.
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In the past year, two men, both from Santa Fe, fell sick with a disease that wiped out millions in the Middle Ages and is now synonymous with "scourge": plague.
Fortunately, the U.S. has only a handful of cases a year of plague, which is now a treatable bacterial infection spread by the fleas on rodents and animals like squirrels, cats, and dogs.
And while there are pockets of infected rodents all across the western U.S., New Mexico seems to bear the brunt of it: In 2009, it saw six of the eight cases nationwide. (The others were in Utah and Illinois.)
Thanks to the measles-mumps-rubella (MMR) vaccine, the number of measles cases in the U.S. plummeted from 3 to 4 million a year in the early 1960s to a few dozen in the early 2000s.
But the highly contagious viral infection is creeping back among unvaccinated children and adults. In the first half of 2011, there were 118 cases, 13 of them in New York City. Although experts attribute most cases to travel (especially to Europe), several cases were contracted locally.
There has also been a bump in mumps; roughly 1,500 of the country's nearly 2,000 cases occurred in the Empire State.
Oklahoma is "where the wind comes sweeping down the plain" (according to Oscar Hammerstein). It's also where the ticks that carry the bacteria responsible for Rocky Mountain spotted fever (RMSF) thrive.
Despite its name, the fever's hot zones are actually scattered across the South. In 2009, a third of the nation's 1,815 cases were in Oklahoma (342) and North Carolina (255).
While less well-known than Lyme disease, RMSF is the most lethal tick-borne disease in the U.S. The infection, signaled by a distinctive spotted rash, kills within two weeks in 10% to 25% of cases if not treated with antibiotics.
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About 150 cases of leprosy—that stigmatized disease of antiquity—pop up in the U.S. each year. In states with the highest incidence, California and Hawaii, many cases are due to travel from areas like Asia, where leprosy (now called Hansen's disease) is common.
Not so for the Lone Star State, which sees a dozen or more cases each year. Texans face a leprosy hazard besides travel: contact with armadillos. In parts of Texas and other Gulf states, up to 20% of armadillos carry leprosy. It's believed that people can contract the disease, which is treatable with antibiotics, by hunting and wrestling these animals, and even gardening in soil where they dig.