[MUSIC] People use the word epidemic all the time in a casual way. You'll hear people talking about a heroin epidemic, or a crime epidemic. There are epidemics of hate, of online hate, of violence, bullying, anger. But those aren't the kinds of epidemics we're going to be studying in this class. We're going to focus on health-related epidemics, and more specifically, on epidemics of highly contagious infectious diseases like the flu, Ebola, and HIV. So what does epidemic mean exactly? Well, the medical definition of an epidemic is, the occurrence in a community or region of cases of an illness, specific health-related behavior, or other health-related events clearly in excess of normal expectancy. There's a lot there, right? So let's break this down into ordinary language. As you saw, according to the technical medical definition, an epidemic is concerned with health, with health related behaviors and events like cancer, HIV, Ebola, or food poisoning. As opposed to behaviors and conditions that are primarily social, and are not mainly about health, like hate, graffiti, or crime. Then within the health category, we find a set of diseases and health conditions that we're focusing on, contagious infectious diseases. Next week, Amish will be teaching you about exactly what a contagious infections disease is. So now that we know what kinds of epidemics we'll be studying, let's return to our original question. What does epidemic mean? And let's go back to that medical definition. The occurrence in a community or region clearly in excess of normal expectancy. So the term epidemic is focused on how much of a disease is occurring. And a disease becomes an epidemic when we a have more cases than we would expect, more than is normal for that place. Essentially, when we have too much of it. Now, I can hear some you thinking, what do you mean too much? Isn't any case of Ebola or AIDS or even the flu too much? Well from an individual perspective, sure. No one wants to get sick and any doctor will try to help a sick patient get better. But the word epidemic comes from the field epidemiology, which is the core public health branch of medicine. Epidemiologists study the patterns of health and disease across human populations and communities, rather than in individuals. And the field of public health is concerned primarily with promoting community or population well-being, rather than with individual well-being. So epidemics are important in the field of public health because they represent the point at which a disease goes from being a threat to individual well-being to being a threat to the well-being of an entire community. Try picturing it this way. We could think of the spread of an infectious disease as a continuum with three general stages. At the low end of the continuum are the normal number of cases for any disease. This is known as the endemic level of the disease in that population. That number is going to be different depending on the disease. So, for a disease like a familiar strain of the flu, you'd expect to see a few cases in any place, at any time. But, for a disease like measles, for which there is an effective vaccination, you'd expect to see only an isolated case here and there in any area that has access to the vaccination. These are just the normal levels of the disease that come and go and don't ring any alarm bells. Now, for some diseases, like smallpox, that have been eradicated, there is no normal level. Even a single case would be too much. An epidemic is the first stage at which a disease becomes of concern from a public health perspective. It means that you see more cases than you would expect for that time and place. The World Health Organization often uses the phrase, community level outbreak, which helps to signal why this is important. It means that the disease has gone from being an individual problem, affecting a few people here and there, to something that's having an impact on a community level. So, in a seasonal flu outbreak or epidemic, you see a lot more people than usual falling ill. For the measles, instead of an isolated case here and there, you now have a cluster of diseases somewhere. Or, for a disease like smallpox that's supposed to be totally wiped out, even a single case would be considered an alarming outbreak that threatens the community as a whole. A note on terminology here, although in popular culture the word epidemic is often regarded as more serious than the term outbreak. The World Health Organization uses the words interchangeably, as does the CDC. A pandemic is the stage at which a disease moves from being a community concern, to a global threat. The 2009 H1N1 flu was considered a pandemic, rather than merely an ordinary seasonal flu epidemic because it spread quickly around the world, and it infected a lot more people than an ordinary seasonal flu epidemic. Other factors that contributed to the concern over this virus were that it was a novel flu virus, so no one had immunity to it. And that it was producing an unusually high number of cases and even deaths among otherwise healthy young people. So we have what amounts to three general stages in the spread of a disease. The normal or endemic stage, in which it affects individuals. The epidemic or community level outbreak stage, in which it affects communities. And the pandemic stage in which it affects the whole world. One reason that this is important is because of the way that we respond at each of these different stages. So in the normal, or endemic stage, just as the disease is individual, so is the response. Individual doctors treat individual patients. But when these cases constitute an outbreak of community level concern, other responses kick in. Doctors are required to report infections diseases to public health authorities. And when an outbreak is identified, those public health authorities begin to react with response measures designed to dampen the spread of the disease and protect the community. Elizabeth will be teaching you about these kinds of local measures in week four of this course. Similarly, if the outbreak becomes alarming enough, then international health actions will be triggered to try to protect the rest of the world from being immersed in a pandemic. In week three of this course, Ryan will talk about these international measures. One last thing, keep in mind that there are a lot of other ways you could divide up this continuum of disease spread and there's not one right way to do it. So for example, the World Health Organization identifies six stages of influenza pandemics and responses. What's important is just to recognize this progression, however you categorize the individual points along the way. As we move forward with this class on epidemics, pandemics, and outbreaks, you should take away two fundamental features of an epidemic. First, what is it that is epidemic? For purposes of this class, we're focus on contagious infectious diseases. And second, what makes it epidemic rather than normal? That there are so many cases, that the disease now poses a threat to community well-being rather than to just individual health. These characteristics are important because they help us understand how epidemics develop and also help us respond effectively to them, so that outbreaks of dangerous diseases like Ebola, AIDS, SARS and whatever is coming next don't become global pandemics. [MUSIC]
