Over the last century, the world has learned what a truly awful flu season is like

A century after one of history’s most catastrophic disease outbreaks, scientists are rethinking how to guard against another super-flu like the 1918 influenza that slaughtered tens of millions as it swept the globe in mere months.

There’s no way to predict which strain of the shape-shifting flu virus could trigger another pandemic or, given modern medical tools, how bad it might be.

But researchers hope they’re finally closing in on stronger flu shots, ways to boost much-needed protection against ordinary winter influenza and guard against future pandemics at the same time.

Here’s a look at flu pandemics over the past 100-plus years:

Spanish flu (1918-19)

A particularly virulent form of influenza swept around the world in the closing months of World War I.

As much as 40 percent of the world’s population became ill. Unlike other pandemics, this flu primarily affected healthy adults.

The highest incidence and mortality rates were among adults 20 to 50 years old.

The majority of deaths were caused by complications — bacterial pneumonia infections caused by the virus.

In some cases, people who felt fine in the morning developed symptoms throughout the day and died by nightfall.

Death tolls: 50 million worldwide, 675,000 U.S.

Asian flu (1957-58)

This flu virus began in Guizhou, China, spread to Singapore in February 1957 and the U.S. that summer, then spread rapidly when children returned to school that fall.

The worst seemed to be over by the end of that year, but a second wave exploded in early 1958.

Death tolls: 1.5 million worldwide, 69,800 U.S.

Hong Kong flu (1968-69)

This flu was first detected in Hong Kong in early 1968, and it spread to the U.S. in September and became widespread in December.

Most of the fatalities were people over the age of 65.

The same virus returned in 1970 and 1972. It’s theorized that patients who had the Asian flu had some immunity to the Hong Kong flu.

Death tolls: 1 million worldwide, 33,800 U.S.

Russian flu (1977-78)

First detected in northern China in May 1977, it then spread to the Soviet Union. In January 1978, U.S. schoolchildren and military recruits began falling ill.

Nearly all victims of this strain were children or young adults.

Researchers think earlier pandemics gave many people immunity to this outbreak.

Death tolls: 1 million worldwide, negligible in U.S.

Swine flu (2009-10)

The H1N1 flu was first detected in Mexico in March 2009 and quickly spread to the U.S.

Researchers found the strain was a genetic combination of types of influenza found in humans and in pigs.

The virus was declared a global pandemic June 11, 2009. President Barack Obama declared it a national emergency on Oct. 25.

H1N1 mostly affected young people. A massive effort to vaccinate 80 million Americans — mostly schoolchildren — helped reduce the impact.

Death tolls: 18,300 worldwide, 2,710 in U.S.