Scientists and health experts at the World Health Organization and Centers for Disease Control are constantly on guard against diseases that could sweep through the human population. Which pandemics are they most worried about, heading into 2014?
The strain of influenza commonly called bird flu is technically HPAI A(H5N1), a specific highly pathogenic strain. It's common in avian populations, but can be transmitted to humans. This is probably the most serious pandemic threat on Earth due to several factors.
First, its reservoir population (animals that host the disease and spread it to other species) is huge and widespread, found on poultry farms and in live bird markets. Second, when humans are infected, chances are they will die. The WHO reports a 60 percent mortality rate.
The worst part about H5N1 is that it mutates quickly and easily, incorporating DNA from other influenza strains it may encounter (in individuals infected by multiple strains).
Currently, H5N1 isn't passed from human to human (or does so very rarely), but studies have shown that just a few mutations could give it this ability. If this were to occur, the resulting pandemic would have horrific potential to infect millions. There is a vaccine against it, but the virus could mutate such that the vaccine is rendered ineffective. For now, H5N1 has been limited to a few dozen cases in humans per year.
This influenza strain first infected humans in 2013. It is not as virulent as H5N1, and the mortality rate isn't as high. However, it has caused a great deal of concern among public health authorities. No one is quite sure why H7N9 suddenly started infecting humans. Our limited understanding of it means we have fewer defenses against it.
There is some evidence that H7N9 already has the ability to be transferred between humans. Research with ferrets (which can transmit respiratory viruses to each other in a similar fashion to humans) shows ferret-to-ferret transmission, and in the Shanghai outbreak it appears that some victims may have transmitted it to their family members.
Nipah is one of several viruses hosted by bats and flying foxes (others include Marburg Virus, Ebola, and Hendra Virus). It is most often spread when pigs eat fruit contaminated by bat saliva or urine. The pigs then spread the disease to humans. During a 1999 outbreak in Malaysia, 1 million pigs were culled to prevent further infections.
Nipah seems to mutate rapidly, moving from encephalitic symptoms (brain inflammation, often leading to death) to respiratory symptoms in later cases. The respiratory form enables human-to-human transmission. Many outbreaks have a mortality rate of 90 percent or more — 2011 outbreak in Bangladesh killed 21 children. If you want an idea of what a widespread Nipah pandemic might look like, the virus in the movie Contagion was modeled on- it.
MERS (Middle East Respiratory Syndrome) is a coronavirus, named for the spiky proteins that stud its surface. It's closely related to SARS, which caused a major scare in Toronto and other cities in 2003, after outbreaks in China the previous year. MERS also seems to be hosted by bats, possibly using camels as an intermediate before transmission to humans. MERS is already capable of human-to-human transmission, and new cases continue to emerge, suggesting MERS won't be a "Middle East problem" for much longer.
Methicillin-resistant Staphylococcus aureus is a strain of bacteria that has evolved resistance to a wide range of antibiotics, making it extremely difficult to treat. MRSA is potentially deadly – in fact, it is responsible for more deaths than the viruses on this list combined. It can be controlled with proper sanitation and isolation techniques, and is not more virulent than other bacteria strains. However, it is so difficult to treat that "outbreaks" can occur in prisons, hospitals, schools, and among other vulnerable populations. New strains have shown resistance to additional antibiotics, which raises the possibility of a bacterial infection that is literally untreatable. In fact, such cases already occur with some regularity.
Centers for Disease Control. "Middle East Respiratory Syndrome."
Hesman Saey, Tina. "Year in Review: a Double Dose of Virus Scares." Science News, Dec. 21, 2013.
USAID. "Emerging pandemic Threats Program."
Photo: Irwin Fedriansyah/AP