Have you experienced extreme cold ? The sensation of cold water dumped over your shoulders ? Have you felt compelled to donate to ALS ? Sufferers report additional symptoms such as Likes, Upvotes and Diggs. If you've suffered these symptoms in the past year, you may have been exposed to the " Ice Bucket Challenge". Don't panic, but do see a professional.

How was it transmitted ?

The "Ice Bucket Challenge" was transmitted through social media and video sharing websites. A victim would usually attempt to spread it to at least three other people, usually after the victim has experienced the most violent symptom of the disease. People could also passively catch the disease, taking on the Ice Bucket Challenge apropos of nothing, and beginning a whole new outbreak. The biggest contributors to passive spread are individuals to whom we shall refer to as "Celebrities".

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How infectious was it ?

There is one simple statistical tool which helps researchers to gauge how infectious an outbreak is, called the "Basic Reproductive Number", also known as R0. It basically tells you the number of new cases caused by the sufferer of a disease.

So in the case of the "Ice Bucket Challenge", the R0 should be 3. Because each sufferer nominates 3 people. However, just like any other disease, just because people have been exposed to the disease doesn't mean they'll become infected.

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Researchers at the University of Hong Kong focussed their studies on how quickly the "Ice Bucket Challenge" spread between celebrities by watching compelling documentary evidence placed on YouTube.

Shown below is the diagram of how a particularly grisly index case, Mark Zuckerberg, was the epicenter for and outbreak of the disease. He managed to spread to three people, but as you can see, not everyone was so infections. Some individuals, like Ryan Seacrest, spread the disease to two people. Others, like Macklemore, only spread to one other person. Steven Colbert was a dead end for the disease, spreading it to no-one.

In practice, the basic reproductive number for this outbreak averaged around 1.43. For comparison, the basic reproductive number for Ebola is about 1.5.

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However, some people were more likely to spread the disease than others. Sufferers with a High Net worth, such as Sheryl Sandberg, Bill Gates and Mark Zuckerberg tended to be more likely to spread this disease.

The disease itself moved very fast, with symptoms emerging, on average, 2.1 days after exposure. This rapid turnover time can explain how this outbreak moved so quickly.

Should I be frightened ? Is it time to crack eachothers heads open and feast on the goo inside ?

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Slow down, it isn't all bad. In fact some would argue that the "Ice Bucket Challenge" produced some beneficial side effects. It raised over $98m for ALS research, and other charities have been working on their own variants of the disease to help them raise money.

Some experts have observed that it may be seasonal, and that winter has forced the outbreak to become dormant. We may expect a re-emergence in the summer. Others believe that it's extinct, but that some other related virus may emerge next summer. In the meantime, the best advice is to be on your guard, and to implement travel restrictions for people of high net worth. Knowing is half the battle.

Transmissibility of the Ice Bucket Challenge among globally influential celebrities: retrospective cohort study BMJ 2014; 349 doi: http://dx.doi.org/10.1136/bmj.g7… (Published 16 December 2014)