Original question asked by Rhiannon: “How likely is a zombie apocalypse IRL?”

I think that with good anti-disease countermeasures, a full blown apocalypse is unlikely. The quickest countermeasure to implement (and the oldest) is quarantine, where travel is restricted and those who might be infected are isolated. We’ve seen something of the likely response to a serious disease outbreak when swine and avian flus have become a concern; security cameras have already been developed to look for elevated body temperature associated with infection.

Characterisation and analysis of the pathogen (infectious disease causing organism) would be a priority since it allows treatment and vaccination (vaccination allows the immune system to learn how to destroy the pathogen without developing the full disease). As the science and techniques improve, we keep getting faster at finding and analysing pathogens, which is a reason to be cheerful.

Once the disease had established itself, it would become treatable via recovery serum. Recovery serum is made from the blood of someone who has recovered from a disease and contains antibodies - proteins which prevent pathogens entering cells, and target the pathogen for destruction by the immune system.

Antibodies are also made by the body in response to vaccination and can help an infected patient recover more quickly and safely. Therefore as long as some people had a degree of immunity, and produced antibodies to the pathogen (and we were able to harvest their blood) the infected could be treated (this turns up in the book and film adaptations of I am Legend).

The power of these countermeasures is demonstrated by smallpox, a major public health issue of our relatively recent past, since it was not only deadly but scarred survivors for life. Smallpox was wiped out in the late 20th century, and now only exists in a small number of research labs. The extinction was due to good monitoring of the disease’s spread; and vaccinating anyone who might be exposed to it, which led to herd immunity. Herd immunity is where an infected person is unlikely to interact with someone who hasn’t already been exposed to or vaccinated against the disease. This means that the disease can’t continue to spread and all the remaining infected individuals either die or recover; wiping out the disease.

A zombie agent therefore would have to rapidly change to evade the vaccine. This rapid change is how the flu and HIV have survived our attempts to vaccinate them into extinction. These mutations would probably stop the pathogen being a zombie agent because the mutated strains of the pathogen probably would not cause zombism. Making a person a zombie means changing quite a lot without immediately killing the host, so even small changes to the pathogen could stop it turning people into zombies.

A key to the countermeasures to a zombie pathogen is supplying medical aid to less economically developed countries, since if a zombie pathogen emerges, we want to know about it before it spreads, so having medically trained staff all over the world would be sensible. This would also allow us to fight the outbreak wherever it occurs, so we don’t have to fight it on a more global scale (so donate to the Red Cross Now!). Diseases (and hence any zombie pathogen) are often spread by poor hygiene and lack of health education, so foreign aid generally is also a good long term strategy since it reduces the ability of a pathogen to spread.

Films and games seem to also offer hope of survival; given that untrained civilians seem to be able to outfight or outsmart zombies, the police and military being overwhelmed seems unbelievable. It seems reasonable that so long as the “living” had any advantage (including the zombies attacking each other), it would be only an unusual pandemic rather than an apocalypse.

It has been pointed out that some other factor might be necessary, such as in John Wyndham’s day of the triffids where the population is already blinded before the zombies (or in this case carnivorous plants) arrive.

It should also be pointed out that widespread zombism is possible if the pathogen didn’t produce symptoms for a long time whilst still being able to spread. It is also possible to be infectious without ever suffering symptoms, the best example of this was “typhoid Mary”, a woman called Mary Mallon who gave multiple people typhoid (a nasty salmonella infection) without ever suffering from any symptoms. This would allow widespread infection, but again I suspect that the “living” would still have a military advantage.

This is not to say that socioeconomic or political upheaval would not happen (think of the long-term effects of short periods of rioting), in fact it has been suggested that most of the violence would not happen from the zombies. Popular culture is rife with ideas about what the uninfected might do to each other in the event of a zombie outbreak. This concept is discussed in this video from PBS Game/Show.

All likely parasites seem to make killing yourself more likely than killing others; this is probably because it’s better for the ability of the disease to spread to new hosts, since we don’t want to interact with people who are aggressive. (Check out this cartoon)

All things considered, a pathogen is unlikely to evolve naturally into or continue to be a “zombie agent” - so we would have to deliberately make it ourselves!