LONDON , England ( CNN ) -- -- The World Health Organization has called the swine flu outbreak spreading around the world a `` public health emergency of international concern . '' Swine flu is usually diagnosed only in pigs or people in regular contact with them . Health workers worldwide are racing to prevent what may potentially become a pandemic . An influenza pandemic occurs when a new virus appears against which the human population has no immunity , according to the WHO . Christine Layton , a public health expert who specializes in influenza at research institute RTI International , told CNN the swine flu has `` pandemic potential . '' `` Unlike the avian flu that people were concerned about a few years ago , a lot more cases are occurring in a lot more different places , '' she said . `` The mortality rate is lower with swine flu , but it seems to be cropping up in a lot more different places . '' See photos of the outbreak in Mexico » . Previous influenza pandemics have been deadly . According to current projections , a pandemic today could result in up to 7.4 million deaths worldwide , the WHO says . Since 1900 , three pandemics have occurred , according to the U.S. Department of Health and Human Services . Spanish flu was the worst pandemic of the 20th century . Up to 40 percent of the worldwide population became ill when it occurred in 1918-1919 . The WHO estimates the Spanish flu resulted in upwards of 50 million deaths -- or more deaths than those during World War I . A virus as severe as Spanish flu has not been seen since . Although 10 times deadlier than other pandemics , Spanish flu was far less contagious than diseases such as measles or chicken pox , according to Harvard epidemiologists Christina Mills and Marc Lipsitch , who carried out a study in 2004 . In 1957 , another influenza pandemic surfaced . The 1957 pandemic was known as the Asian flu . Watch Dr. Gupta report from the epicenter of the outbreak » . It was sparked by the H2N2 strain and was first identified in China . There were two waves of illness during this pandemic : The first wave mostly hit children , while the second mostly affected the elderly . It caused about 2 million deaths globally . But the medical community was able to identify that pandemic more quickly because of improvements in scientific technology , according to the U.S. Centers for Disease Control . The most recent pandemic occurred in early 1968 when a flu pandemic surfaced in Hong Kong . About 33,800 people died between September 1968 and March 1969 -- making it one of the mildest pandemics of the 20th century . While no pandemics have surfaced since 1968 , other pandemic `` threats '' have occurred in the 20th century , including the 1976 `` killer flu '' ( later named `` swine flu '' ) threat in the United States , which led to a mass vaccinations amid fears it was related to the Spanish flu virus . The most recent pandemic threats occurred in 1997 and 1999 . Hundreds of people became infected with the avian flu virus , or bird flu , which killed six people and infected hundreds . This virus was different as it moved from chickens to people , rather than moving through pigs first . Around 1.5 million poultry were slaughtered in Hong Kong to contain the threat . The rise of global air travel has raised the ability of disease to spread more rapidly than ever before . Severe acute respiratory syndrome , better known as SARS , was the first severe transmissible disease to hit the globalized world when it hit in 2003 . But outbreaks like SARS , which saw the application of control measures like quarantine , travel restrictions and fever checks at airports , have helped the health community better prepare for emergencies . Sources : World Book Online , U.S. Department of Health and Human Services .
WHO raises pandemic alert level to Three ; level Three represents global pandemic . 6 pandemics have occurred since the 20th century . Spanish flu was the worst pandemic of 1900 .