![]()
Tara C Smith summarises the epidemiology and pathology of zombie infections and calls for research and funding to prevent a zombie apocalypse Zombies—also known as walkers, Zed, Zs, biters, geeks, stiffs, roamers, Zeke, ghouls, rotters, Zoms, and runners—have become a dominant part of the medical landscape. Zombie expert Matt Mogk defines a zombie with three criteria: it is a reanimated human corpse; it is relentlessly aggressive; and it is biologically infected and infectious.1 But Mogk notes that this definition has been altered by the recognition of “rage” zombies, which are infected but still alive. They are more closely related to vampires infected with the contagious bacterium Bacillus vampiris.2 Here, I review zombie biology and epidemiology. Descriptions of zombies date back to the 1500s.3 Haitian zombies are probably the best described, often thought to be controlled by practitioners of voodoo.4 They may have been created via a neurotoxin, typically described as tetrodotoxin, which puts the victim in a sleep-like state.5 These voodoo or chemical zombies seem to be unrelated to the current wave of epidemics, which began with the first documented outbreak in 1968 (fig 1⇓).6 The modern outbreaks are thought to be infectious in aetiology and transmissible by bite. The Solanum virus is the most extensively studied infectious cause of reanimated zombies.7 8 9 It has caused outbreaks around the world but does not have an identified reservoir in nature.8 It has a 100% mortality rate, and zombification is certain in anyone exposed to an infected person. Solanum infection is universally fatal in all animals tested or observed, indicating that zoonotic transfer to humans is an unlikely origin.8 One anecdotal report linked infection to the looting of underwater settlements in the Three Gorges Dam in China.9 A child emerged, bitten, but his fishing partner did not surface at all. The dam was created by relocating over 1.24 million residents and flooding their former villages.10 Zombie expert Max Brooks thinks that Solanum virus infection has occurred for thousands of years and is now emerging because of urbanisation and the interconnected nature of commerce and travel.8 Whether this virus is the cause of the current outbreak in the United States has not been tested.11 12 Although reanimated zombies have been documented for potentially millennia,8 rage zombies seem to be a more recent phenomenon. These date back to at least 1973, when the weaponised Trixie virus was accidentally released in Pennsylvania.13 This virus was also responsible for a 2010 outbreak in Iowa.14 In both cases a military plane carrying the virus—a modified form of rhabdovirus—crashed into a small town’s water supply. Townsfolk were infected via ingestion of the virus and possible airborne spread. Infection caused the inhabitants to develop a murderous rage. The outbreak was quashed by a controversial military intervention and forced quarantine, but at a high cost of lives (fig 2⇓). Infected people may remain. A 2002 epidemic of rage zombies decimated London, UK, leading to quarantine of the island of Britain.15 This outbreak was traced back to the release of a genetically modified virus derived from Ebola that was tested in captive chimpanzees.16 An animal rights activist was bitten while trying to liberate the chimpanzees from a laboratory in Cambridge. Because the incubation period of the virus is short (seconds), and the victims are quick and strong, the infection rapidly spread across the country, necessitating a complete border closure (fig 3⇓). Twenty eight weeks into the outbreak, after many of the zombies had died from starvation, a rare asymptomatic carrier of the virus was discovered, beginning a second wave of infections.17 Symptoms of infection tend to be fairly uniform, regardless of the nature of the pathogen, but the incubation period is highly variable, with time to development of symptoms ranging from seconds15 16 17 to hours or days.11 12 13 14 18 Infected people may clinically die and reanimate,6 9 11 12 19 20 21 or they may remain alive but with the same aggressive tendencies and taste for human flesh as reanimated zombies.13 14 15 16 17 Other symptoms may include a shambling gait, tendency to moan, loss of dexterity and prior personality traits, and the eventual rotting of flesh. In rare cases zombies may be highly intelligent, self aware, and lacking in the typical tendencies to bite and eat flesh.13 14 What unites many outbreaks is transmission via bite. Other ways of becoming infected include insect vectors,22 23 animal bites,15 19 24 and a ubiquitous condition whereupon everyone reanimates as a zombie upon death.11 12 22 24 Insect vectors seem to be rare, but include bedbugs in an island outbreak off the coast of South America23 and mosquitoes as a vector of the modified Kellis-Amberlee virus.22 Others types of pathogen have been reported. The causative agent of the zombie outbreak discovered retrospectively on the Titanic was a weaponised form of the bubonic plague bacterium, Yersinia pestis.25 Cordyceps fungus has been documented as the cause of a diverse set of zombie phenotypes,26 spread via bite or by spores released from dead hosts. A mutated strain of bovine spongiform encephalopathy—a prion infection—led to a US outbreak of zombiism.18 In other cases, a combination of pathogens has been identified as necessary and sufficient to cause zombification.20 21 Because of the rapid onset of zombie outbreaks and their society destroying characteristics, prevention and treatment are largely unexplored. Severing the bitten area from the body has proved successful in some cases11 12 but is not universally preventative, and it is sometimes impossible owing to bite location or the speed of viral incubation.15 17 Vaccines have been difficult to study because of the associated cost and the inadequacy of many laboratories to provide proper containment of zombie pathogens or infected zombies, as well as the diversity of zombifying agents. Vaccine hesitancy may make it difficult to achieve society-wide uptake of a zombie vaccine.27 Even if an effective treatment were developed, it may need to be taken perpetually to prevent the affected person from reverting to zombiism.28 More research in this area is sorely needed. Zombie outbreaks are expensive, difficult to control, and have deleterious effects on the stability of life. Quarantines often fail or are unable to accurately contain people who may be infected but not yet symptomatic, resulting in a de novo outbreak within the quarantine facility.13 14 17 Resources often become depleted, and zombies can over-run cities or entire countries in days to weeks.9 15 Complete containment of the zombie infection is rarely achieved. A notable exception may be the 2004 London outbreak, where the government stemmed the infection with a military response, and remaining zombies were used for labour and entertainment.29
