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The deadliest strains of viruses often take care of themselves — they flare up and then die out. This is because they are so good at destroying cells and causing illness that they ultimately kill their host before they have time to spread.

But a chicken virus that represents one of the deadliest germs in history breaks from this conventional wisdom, thanks to an inadvertent effect from a vaccine. Chickens vaccinated against Marek’s disease rarely get sick. But the vaccine does not prevent them from spreading Marek’s to unvaccinated birds.

“With the hottest strains, every unvaccinated bird dies within 10 days. There is no human virus that is that hot. Ebola, for example, doesn’t kill everything in 10 days.”
In fact, rather than stop fowl from spreading the virus, the vaccine allows the disease to spread faster and longer than it normally would, a new study finds. The scientists now believe that this vaccine has helped this chicken virus become uniquely virulent. (Note: it only harms fowl). The study was published on Monday in the journal PLOS Biology.

This is the first time that this virus-boosting phenomenon, known as the imperfect vaccine hypothesis, has been observed experimentally.

The reason this is a problem for Marek’s disease is because the vaccine is “leaky.” A leaky vaccine is one that keeps a microbe from doing serious harm to its host, but doesn’t stop the disease from replicating and spreading to another individual. On the other hand, a “perfect” vaccine is one that sets up lifelong immunity that never wanes and blocks both infection and transmission.