Contagiousness

Using data from the known cases of MERS, the researchers found that, on average, someone with the disease is likeliest to infect at most, only one other person. SARS patients were likely to infect three others. That level of contagiousness made it a pandemic.

While people had no knowledge of modern germ theory in the year 1000, they were well aware of the contagiousness of diseases.