Seen Contagion yet?

It's pretty scary. A new epidemic disease comes out of nowhere and starts killing everyone. It infects the brain - victims suffer seizures, or fall into a coma, and die. It spreads like wildfire. Humanity's only hope lies in Lawrence Fishburne and Kate Winslet.

Luckily, that's fiction. But only just.

In the movie, the killer bug is called "MEV-1", but it might as well have been called the Nipah virus, because it was closely based on a real disease of the same name. So much so that this post about Nipah contains movie spoilers.

The Nipah Virus came to the world's attention in late 1998. There was an outbreak of a severe fever accompanied in many cases by encephalitis (viral infection of the brain) in Malaysia and Singapore. 276 patients were recorded. 40% of them died.

In the initial outbreak, there was probably no person-to-person transmission of the virus. Rather, only people who came into contact with Malaysian pigs - mainly farmers and butchers - caught the disease. Over a million pigs were culled in 1999 to try and contain the outbreak, and this seemed to be effective.

But since then, there have been several other smaller Nipah outbreaks in Asia, one almost every year in fact. In some of these, person to person transmission has been detected, notably in Bangladesh and India. The fatality rate in these more recent outbreaks has also been higher (70-90%). Luckily, unlike in the movie, it doesn't seem to be very contagious - so far. Most years have seen only 10 or 12 cases. But who knows what the future holds?

The virus is distantly related to measles, but is much more severe. Symptoms can begin anywhere from 4 days to 2 months after infection, but generally within 1 to 2 weeks. More recent outbreaks seem to have a shorter incubation period. The symptoms include fever, headache, vomiting, seizures, muscular jerks, and altered consciousness (confusion, coma).

Even after the initial infection is over, a minority of patients (4-8%) later suffer a relapse encephalitis. The virus seems able to remain dormant in the body before re-emerging to infect the brain again. Survivors may suffer neurological problems such as epilepsy, movement disorders, fatigue, and others. This is especially common following relapse encephalitis.

Where did it come from? It turns out that various strains of Nipah-like viruses are common in certain bats that inhabit various Asian countries, specifically fruit bats of the Pteropus genus, aka "flying foxes". The bats don't get sick, but infected bats are highly contagious, excreting the virus in their urine.

The virus seems to have made the leap into humans not once but several times, from different kinds of bats. Each outbreak could represent a new crossover event. Often there was an intermediate animal host, such as the domestic pigs in Malaysia .

Nipah is a classic zoonotic disease - it jumps from animals to humans. Zoonoses are scary for two reasons. They're new to humans, so humans haven't had a chance to develop immunity. And they may be especially deadly, because they haven't evolved not to be deadly to us.

Viruses and bacteria don't actually want to kill you. They want you alive, so that you can keeping breathing, walking, having sex, and otherwise spreading them. So pathogens tend to evolve to be less lethal to their primary hosts. Unfortunately, that's only good news if you are the primary host, and in the case of zoonoses, we're not. Bats don't get sick, but we do.

Lo, M., & Rota, P. (2008). The emergence of Nipah virus, a highly pathogenic paramyxovirus Journal of Clinical Virology, 43 (4), 396-400 DOI: 10.1016/j.jcv.2008.08.007