Find out how the seasonal flu can become a killer cold.

For most people, flu is a seasonal nuisance, announced by a fever, a headache, and a sore throat. A few days to a week and you're over it. However, the virus can be a lot more serious. Influenza, as it's properly known, causes up to half a million deaths a year worldwide. And less than a century ago it became a global catastrophe, striking a fifth of the planet's population and claiming more than 50 million lives. Experts say we can expect a similar outbreak in the future.

Among the most common contagious human diseases, influenza usually strikes in winter, spreading through airborne droplets and particles emitted in coughs or sneezes. Between 5 and 15 percent of the population are affected by these annual epidemics.

Human influenzas are divided into two groups, A and B. Influenza A contains the key strains, which are distinguished by spikelike features known as antigens. The genetic makeup of these viruses allows for frequent mutations. That's how they keep managing to sneak past our immune systems and why new flu vaccines are needed each year.

These vaccines are important for the elderly and sick, who have a much higher risk of flu-related health complications such as pneumonia, a potentially fatal lung infection. More than 90 percent of deaths linked to influenza are in people over 65.

We can blame birds—particularly waterfowl—for that's where flu first originated. These birds act as reservoirs for Influenza A and are thought to be its source in all other animals. Human influenzas, once established, are usually familiar enough for our bodies to deal with, despite their mutations. But occasionally something totally new comes along—and finds us sitting ducks.

Historic Outbreak

It happened, infamously, in 1918, with the outbreak of Spanish flu (so named because Spanish newspapers were among the first to report it). Known as a pandemic because of its global reach, Spanish flu spread as far as the Arctic and remote Pacific islands. It killed more than 10,000 people a week in some cities. The victims, many of them young adults, suffocated as their lungs drowned in bloody fluids. About 5 percent of those infected died. Current estimates suggest between 60 million and 100 million people died worldwide.

Today, scientists think Spanish flu was a bird flu, or avian influenza. In two subsequent lesser flu outbreaks in 1957 and 1968, an avian flu combined with a human flu to create a hybrid strain that was able to infect people. For this to happen, the two viruses needed to coexist in the same animal species. Pigs are seen as the likeliest candidates.

Studies suggest that Spanish flu was so lethal because it somehow crossed into people as an animal virus, without the genes from human strains that our immune systems were expecting.

The same happened with the latest bird flu to affect humans. Spread by wild birds, the H5N1 virus jumped from chickens to people in 1997. First recorded in Hong Kong, nearly all infections to date have come through close contact with poultry.

Swine Flu Outbreak

In spring 2009, an outbreak of swine flu (officially called H1N1) killed more than a hundred people in Mexico, raising fears of a possible pandemic. Cases were soon reported across North America, and in October 2009, the first H1N1 vaccines became available.

Swine flu is caused by an Influenza A virus and affects pigs year-round. The disease is able to jump to humans when a strain develops that is a mixture of animal and human versions of the virus. It is usually spread by people who are in close contact with pigs, such as farmers.