Modern transportation, which makes travel easier for humans, also helps dangerous viruses move around the world with human hosts. The first large-scale epidemic of the decade -- the SARS virus- started in Asia in February 2003 and spread to more than 25 countries. A city worker carries buckets of rat poison in China, Jan. 8, 2004. Chinese response to SARS was a rat extermination campaign.
SARS: Prevention
Children taking a ballet class in Hong Kong wear masks to protect themselves from SARS in this April 27, 2003, file photo. According to the CDC, severe acute respiratory syndrome (SARS) sickened 8,098 people worldwide, killing 774.
SARS: Masking Fear
A Taiwanese guard in plain clothes wears a mask to protect himself from SARS, as he wipes sweat off the face of an Honor Guard on duty at the Martyrs Shrine, a famous tourism spot, Tuesday, April 29, 2003, in Taipei.
WEST NILE VIRUS: Warning
A West Nile virus warning sign at the Madison County Health Department's waiting room in Florida. First identified in 1937 in Africa, West Nile fever was found in New York in 1999. Since, then it has spread throughout all 50 states.
West Nile: Spraying for Health
City of Dallas environmental workers spray for mosquitos in Texas. According to the CDC, the virus is spread by mosquitoes and is a seasonal epidemic in North America that flares up in the summer and continues into the fall.
West Nile: Testing for the Virus
A blood test tray for West Nile virus. West Nile virus is a potentially serious illness, although 80 percent of those infected show few, if any symptoms. According to the CDC, one in 150 infected people develops a severe illness that could include coma and permanent neurological effects.
Bird Flu: On to Humans
Nguyen Cong Thanh, 10, a suspected patient of avian influenza at the National Pediatric Hospital in Hanoi, Vietnam, Tuesday, Jan. 20, 2004. Avian influenza outbreaks among poultry occur worldwide from time to time, but in the late 1990s and early 2000s, the virus began turning up in humans who had close contact with infected birds.
Bird Flu: Poultry Problem
Anchalee Hunsilp, 39, a Thai farm owner, collects eggs at her farm in Supanburi province, Thailand, Jan. 15, 2004. The Word Health Organization has reported 447 human cases of bird flu in Asia, Africa, the Pacific, Europe and the Near East since 2003. Of these, 262 people died. Indonesia and Vietnam have reported the highest number of H5N1 cases to date. Overall mortality rate is 60 percent.
Bird Flu: Testing Positive
Nguyen Thanh Hung, 42, sits on his hospital bed in Bach Mai Hospital in Hanoi, Vietnam, on Wednesday, Jan. 26, 2005. In the United States, from 1997 to 2005, there were 16 outbreaks of bird flu in poultry but there has been none in humans. The most recent human cases occurred in 2007 in the UK and Hong Kong.
H1N1: Race for Vaccine
In this Oct. 26, 2009, photo, Joseph Hartstein receives the H1N1 vaccination at the local health authority in Dusseldorf, western Germany. The H1N1 flu, initially dubbed swine flu, was first detected in the spring of 2009 in Mexico and quickly spread to dozens of countries worldwide.
H1N1: Flu Prevention
A kindergartener wears a mask as precaution against the H1N1 flu. The girl washes her hands before going into a class in Hong Kong, June 11, 2009. H1N1, according to the CDC, is thought to spread mostly from person to person through coughs and sneezes of people who are sick with the flu. A vaccine now is available.
H1N1: Stopping the Spread
In this Oct. 6, 2009, photo, Amparo Martinez, left, watches as her daughter, Sorayo Martinez, 4, is given a dose of H1N1 vaccine in Oregon City, Ore. From April to Nov. 14, 2009, the CDC estimates, there have been between 34 million and 67 million cases of H1N1 leading to an estimated 154,000 and 303,000 hospitalizations. The CDC also estimates there have been 7,070 and 13,930 2009 H1N1-related deaths.