Flu outbreaks

Swine flu is not the first example of a fatal influenza. Worldwide outbreaks have killed millions of people, although they've become less frequent and deadly. Read below to see some examples in recent history.
Spanish Flu (1918-1919)
Flu tents for Spanish influenza
Spanish flu tents in Lawrence, Mass., 1918. (File Photo)
The "Spanish" influenza pandemic of 1918 and 1919 caused about 50 million deaths worldwide. The disease was exceptionally severe, killing more than 2.5 percent of those infected.

Almost all subsequent cases of epidemic influenza have come from descendants of the Spanish flu.

Asian flu (1957-1958)
In 1957, the virus was quickly identified due to advances in technology and a vaccine was produced, according to a North Carolina State University study on flu history. Infection rates were highest among children and pregnant women, while the elderly had the highest rates of death. A "second wave" of the Asian flu developed in 1958. There were about 70,000 deaths in the Untied States.

Hong Kong flu (1968-1972)
This strain caused 33,800 total deaths in the United States, according to a North Carolina State University study on flu history. This virus was first detected in Hong Kong in early 1968 and spread to the United States later that year. Those older than 65 were most likely to die. This virus returned in 1970 and 1972 and still circulates today.

Bird flu (2003-today)

Bird flu
A Czech veterinary worker collected a swan after a 2007 bird flu outbreak. (Michal Cizek / AFP / Getty Images)
Avian influenza, or bird flu, spreads rapidly through poultry flocks and has a mortality rate approaching 100 percent -- and it has become contagious for humans in recent years.

The current outbreaks of highly pathogenic avian influenza have caused 257 deaths since mid-2003, mostly in Southeast Asia, according to the World Health Organization. The disease has also resulted in the death or destruction of an estimated 150 million birds. Control of the disease in poultry is expected to take several years.

Swine flu (ongoing) The number of people killed by the swine flu in Mexico has climbed to 149, up from a toll of 103 over the weekend, prompting officials to shut every school in the country and discuss the possibility of a further shutdown of Mexico City. As of Monday afternoon, 40 cases have been discovered in America, and the federal government declared a public health state of emergency.

Swine flu symptoms are similar to regular flu symptoms, and have been known to cause death and severe illness such as pneumonia in the past.

All the information above comes from the Centers for Disease Control unless otherwise noted.