YOSEMITE NATIONAL PARK, Calif. -- Hantavirus in Yosemite. West Nile virus in 48 states. Even a case of bubonic plague.

"I hear locusts are next," says Cathi Soriano of Seattle, who recently took Yosemite National Park off a road-trip itinerary.

Are we under siege?

Notreally, but the medical victories we've experienced over the past 100years have made Americans forget that such diseases haven't gone away,says David Dausey, director of the Institute for Public Health atMercyhurst Universityin Erie, Pa. "It's unsettling to realize that we'renot entirely safe from these things."

MORE: Yosemite hantavirus warning extends worldwide

The rise of hantavirus andWest Nile virus, neither even recognized in the United States before1993, is making people check their window screens, stock up on bug sprayand rethink travel plans. In Yosemite this summer, hantavirus haskilled three people out of nine sickened. Nationally, West Nile virus isthe worst it's been since the disease arrived on our shores in 1999:more than 3,545 illnesses and 147 deaths as of Thursday.

Extremeweather patterns have played a big role in the two recent outbreaks, andhealth officials worry more such events could be on the horizon becauseof climate change.

Climate cycles very clearly play a part inoutbreaks, says Michael Osterholm,director of the Center for InfectiousDisease Research and Policyat the University of Minnesota inMinneapolis. The question is at what point any given outbreak is beingcaused by climate change or simply normal weather cycles. However, it'sclear that "eventually (climate change) will affect things, but is itnow? We don't know," he says.

At the same time, health officialsfret that the public health infrastructure of laboratories and publichealth workers that tracks and responds to outbreaks is being cut. Thatcould make outbreaks harder to detect and control.

"The federal spigot is not just being cut off, it's being smashed," Osterholm says. "We've got a crash coming. We can see it."

Inthe hantavirus outbreak at Yosemite, weather and a move to provide moreeconomical lodging for winter sports enthusiasts could be behind theillnesses. Three deaths are worrisome but doctors are particularlytaking notice because they came in a tightly focused geographic cluster.

"It'snever happened before," says Pierre Rollin, a chief with the Centersfor Disease Control and Prevention's Special Pathogens Branch.

On a brilliant fall day, it's impossible to get into the park and not hear about hantavirus.

"Theygave me a pamphlet when I came through the main gate, then one when Ichecked in (at Curry Village), and there was one on my bed when I got tomy cabin and one pinned to the wall. And there's one in the bathroom,"says Rebecca Costello of Ambleside, England. "Those people up there,"she says, pointing at rock climbers clinging to the Half Dome are "in alot more danger than I am here.""Now's probably the safest time to behere," agrees Nicole Swedlow, 38, who runs a children's charity in SanPancho, Mexico. "They've got people all over it."

PhillipeBrachais, 30, of Antwerp, Belgium, has no worries about staying nearby."It's only three people dead, and millions come here every year," hesays.

His companion, Fabienne Verwerft, 29, says her only concession was to buy some hand sanitizer before they arrived.

Therehave been some cancellations, but park visitors are still on track toreach the September average of about 17,000 people a day, said TomMedema, chief of interpretation and education at Yosemite National Park.

Butin the background, epidemiologists, wildlife biologists and researchersare catching and testing mice, taking environmental samples andcontacting people who stayed in the area -- those who got sick and thosewho didn't -- to see how their visits differed.

Hantavirus haslong been known in the United States. Every year from 11 to 48 people --mostly in the West and Southwest -- come down with it, and about 33% ofthem die. But never in clusters, as at Yosemite. That's why theNational Park Service has sent letters and e-mails or made phone callsto more than 260,000 overnight guests who stayed in the park since earlyJune to warn them, Medema said. So far, no park workers are known tohave come down with the virus, Medema said.

On Wednesday, the parkbegan a pilot survey to draw blood samples from approximately 100employees to find out if they had been infected with hantavirus in thepast. Because most people who get hantavirus don't have symptoms, it'spossible others have gotten it before but no one knew. Once the pilotsurvey is completed the park hopes to open it to all staffers. This "maymake an important contribution to our knowledge about this rare virus,"Medema said.

The virus is carried by deer mice, which liveacross the United States. People get it by breathing in aerosolized micedroppings and dried urine -- that is, mouse dust -- so victims have tobe in close contact with the mice or their nests. It is not transmittedbetween humans.

The Yosemite outbreak is still a medical mystery,one that dozens of wildlife and health experts are combing the park tosolve. It is known that wet summers give mice ample food, allowingpopulations to climb. Follow that with a mild winter, as last winterwas, and it's a problem.

"It's the adults who survive the winterwho transmit the virus," says Danielle Buttke, a veterinaryepidemiologist with the U.S. Public Health Service in Fort Collins,Colo., who was in Yosemite investigating the outbreak. If the summerthen is especially dry, all those mice are looking for food and homes --which they found in the tent sites at the park's Curry Village.

The first big U.S. hantavirus outbreak in 1993 "was a very wet summer followed by a mild winter," Buttke says.

In2009, in the category of possible plain bad luck, the park addedinsulation to some tents to replace winterized cabins destroyed in amassive rock fall in 2008.Unfortunately, mice love to live ininsulation, says Gregory Glass, a hantavirus specialist and professor ofinfectious disease ecology at Johns Hopkins University in Baltimore.

"Itmay just be one of those situations where, historically, when theydidn't insulate them, it wasn't that attractive for the mice to sit andstay," he said. "But this one little tweak in the design made just thatmuch of the difference for mice."

West Nile has certainly had abumper year because of the weather. The Culex mosquito that carries thevirus prefers breeding in the murky bottoms of drying pools and watersources -- exactly what happens in droughts. And this year drought hasscorched the nation at levels surpassed only twice before, according tothe National Climatic Data Center.

People such as Osterholm worrythat we're gutting our ability to fight such outbreaks when more extremeweather is in our future. With West Nile virus, a mosquito-bornedisease, cutbacks have hit local mosquito abatement districtsnationwide, from the loss of a few staff members to the closing of NorthCarolina's entire mosquito Pest Management Section. CDC's Vector BorneDiseases group, which works on West Nile virus, among other diseases,has lost $14 million in funding since 2005, 38% of its budget.

"It'seasy to count how many police or firefighters you have," Osterholmsays. But no one's counting how many public health people who canrespond to a crisis are losing their jobs. "We're going to wake up onemorning and say, 'There's a big outbreak somewhere,' and people will askwho's responding, and we'll say, 'We don't know -- we don't really haveanyone there.' "

The one episode this summer that didn't appearto be anything other than normal was the case of bubonic plague in7-year-old Sierra Jane Downing of Pagosa Springs, Colo. Doctors believeshe caught it from fleas off a dead squirrel she encountered whilepicnicking with her family.The bacterium that causes plague, Yersiniapestis,is found in rodents and their fleas in many parts of the world,including the United States. Every year five to 15 people catch it,according to the CDC.

Doctors at Rocky Mountain Hospital forChildren in Denver, where she was being treated, quickly realized shehad the plague and gave her the appropriate antibiotics. She went homeafter 2½ weeks in the hospital.