Scientists are not just scared of how Avian, Swine influenza, and HIV/AIDS can bring out harm today. They fear that two deadliest plagues from the past have been connected and have the potential to return which may kill people within 24 hours if mutated as airborne infection.

Past Plagues to Haunt Back

There are two plagues which caused the deadliest pandemics known in history and that may return as new breeds to create future outbreaks. Researchers have found DNA bits from teeth of two German victims killed by the Plague of Justinian about 1,500 years ago and after genome reconstruction, they have found that a strain of Yersinia pestis caused the plague.

Yersinia pestis is the famous bacteria known to have caused the Black Death that killed 75 to 200 million people in Europe and has been determined to have killed millions in Asia, North Africa, Middle East and some parts of Europe during the Plague of Justinian.

"What this shows is that the plague jumped into humans on several different occasions and has gone on a rampage. That shows the jump is not that difficult to make and wasn't a wild fluke," said Tom Gilbert, professor at the Natural History Museum of Denmark noted by NBC News.

Pests are great factors to spread diseases in any nation inhabited by humans such as fleas on dogs or rats, rats themselves and mosquitoes. The bacteria which caused the two plagues may be lying dormant in deserts and arid places, where fleas could survive for long periods of time, that experts see a possible comeback more powerfully than imagined.

Modern Antibiotics

Modern medicine has evolved to produce effective antibiotics against infections caused by Yersinia pestis. However, any modern plague epidemic remains devastating even with available treatments.

"Plague is something that will continue to happen but modern-day antibiotics should be able to stop it," director of the Ancient DNA Centre at McMaster University, Hendrick Poinar said.

But public should be warned that strains and resistances are something possible on every bacteria upon evolution. Such evolution can make these strains airborne after bacteria reaches the lungs and spreads by coughing. This kind of airborne plague can kill people as fast as 24 hours after infection that does devastation to most undeveloped countries.

Notable Plagues Caused by Yersinia pestis

1. Black Death with estimated death of 75 to 200 million and ended by 19th century.

2. Plague of Justinian was a pandemic that afflicted Byzantine Empire prior to the Black Death.

3. Bubonic Plague is a disease connected to the Black Death that can kill infected people within four days.

4. Third Pandemic is a major Bubonic Plague pandemic that began in China then later spread in India which was active until 1959.