Fall is flu season and this year, as usual, federal health officials are asking the public to get their annual flu vaccine shot.

But scientists say there are much more deadly bugs lurking out there that could someday make the jump from local outbreak to a worldwide super pandemic that could wipe out people across the globe. All it takes is a few biological tricks for a microscopic virus to turn into a raging killer like the 1918 Spanish flu virus that killed 50 to 100 million people, or the SARS virus that started in China in 2003 and spread to 37 countries in just a few weeks. It eventually killed fewer than 800 people.

This year, two contagions that are scaring epidemiologists the most are another Asian virus called H7N9, and MERS, or Middle East respiratory syndrome. Both appeared in 2012.

Going Viral: Pandemics in Fiction and Fact

What does it take to make the jump? Medical researchers say nightmare bugs are quick to evolve, resistant to treatment, have lethal power and the ability to spread from person to person, usually through the air.

To fight these viruses, scientists are deploying new tools of genetic screening to identify the evil-doers, as well as old-fashioned public health measures to quickly isolate patients and stop an epidemic’s spread.

"The two most critical things are virulence and transmissibility," said Scott Dowell, director of global disease detection and emergency response for the Centers for Disease Control in Atlanta.

Dowell was part of the CDC team in Bankok, Thailand, that responded to early reports of the SARS outbreak in 2003. Even though that epidemic eventually did not kill as many people as expected, he said the outbreak scared medical experts because of its incredibly fast spread.

New Sars-Like Bug: Pandemic Potential?

"In the thick of it, it wasn't clear what direction things were going to go," Dowell told Discovery News. "It was an impressive and frightening time."

SARS -- severe acute respiratory syndrome -- is a member of the corona-virus family of microbes (same as the common cold). It originated in the farms of China's southern Guangdong Province where it made the leap from farm animals to humans in November 2002. It did that by reassembling its genetic material to take over host cells and replicate.

By April 2003, it had spread throughout Asia and was killing one out of every 10 infected patients. Dowell said the only things that stopped its spread were international cooperation and the ability to quarantine people who got infected.

By summer of 2003, the outbreak was contained after infecting more than 9,000 people.

Today, fears are rising about a new virus, H7N9, that started in poultry in China and infected 130 people in April 2013, killing 44. Luckily, there hasn't been any evidence of human-to-human spread, but researchers at the National Institutes of Health have just begun the first vaccine clinical trial for H7N9 at nine hospitals across the United States.

The CDC plans to "build" the latest version of the bird flu virus to use it in its vaccine.iStockPhoto

In Saudi Arabia, doctors are watching the spread of MERS, another corona-virus that has been linked to four Middle Eastern nations but also has spread to Europe. So far, nearly half of the 114 people who contracted the virus have died.

Dowell says that researchers are slowly making progress in developing drugs to treat some of these respiratory viruses, which are normally immune to anti-biotics. That's due to advancements in treating another virus, HIV, the virus responsible for AIDS.

While the CDC and NIH are engaging in basic research and epidemiology to understand these killer diseases, some experts say there's a big gap when it comes to drug development.

Diseases That Just Won't Quit

"There isn't a lot of incentive on the part of industry to make the major investment, $700 million to $1 billion for each drug," said Anthony Fauci, director of the National Institute of Allergy and Infections Diseases. "There's a big market for Lipitor, or for a Viagra-like drug because the pharmaceutical companies know when they will make the drug there will be hundreds of millions of people taking it."

Fauci says he's been working to develop partnerships between NIH and drugmakers to get ready for the next outbreak or drug-resistant superbug.