I prefer to use the term epidemic to describe outbreaks, anything quickly spreading, and serious diseases that aren’t under control (tuberculosis, for instance). But other media also use it, almost as hyperbole, to describe anything they want you to take seriously.
Autism, for instance, has had a number of campaigns talking about the autism epidemic. Autism rates vary depending on who you ask (Are they talking about undiagnosed estimates? Severe autism? Autism spectrum disorders?) but generally are quoted at about 1-2 per 100 people. With the new DSM collapsing Asperger’s into autism, that number will get higher.
Now, autism is something that’s currently over diagnosed. Many diseases fall under autism spectrum disorder, and it’s the disease du jour to diagnose a troublesome child with.
Compare this with Prosopagnosia, or facial blindness. Currently gaining attention from celebrities, current estimates put prosopagnosia at about 2.5 out of 100—more than autism. According to a Slate article (about a better explanation for why people can’t tell Superman from Clark Kent—it’s good fun, check it out) this makes facial blindness “extremely rare”. Note that most sufferers don’t know they have it, so unlike autism, it’s more likely to be underdiagnosed.
So, how many people out of a hundred have to have a disease before it’s an epidemic? If it’s something “scary”, like autism (autistic people hate being told autism is an epidemic and needs cured, by the way), it an epidemic. Something most people won’t be bothered by? Extremely rare.
What do you think constitutes an epidemic? How many people out of 100 (what percent) need to be affected?