 How at risk do you feel about the possibility of getting communicable diseases like measles or whooping cough? Do you use statistics and news reports to shape your feelings? Or do you do something else? It turns out that when we try to figure out how rare or common a disease is we try to think of people we know or people we have heard of who have the disease. If we can easily think of people who got a disease it seems common. For example, if your child's best friend recently got measles then we tend to feel that we are at risk. On the other hand, if we really have to work at it to think of anyone who has had a disease, that disease feels rare to us, and we feel safer. This process is what psychologists call the availability heuristic. The strange thing is the actual number, the statistics saying how many people get sick don't tend to change this feeling. Numbers might change what we think consciously about risk, but more often it's that availability heuristic that shapes our feelings about risk. This is important as it helps to explain many people's attitudes toward vaccines. 60 years ago most people knew someone personally who had gotten sick with measles or polio or whooping cough. This personal connection with infectious diseases made people feel at risk. It made them want to get vaccinated even though back then vaccines were very new and by today's standards relatively untested. Today most parents don't know anyone who has had any of these diseases. Vaccines have been incredibly successful at protecting people's health and saving lives, but as a result they feel safe and secure. The problem is in the modern world the threat of these diseases comes from people we don't know. We live in a world where sick people can take airplanes and cars and buses and bring distant diseases to our neighborhoods at any time. So even though these diseases are rare they remain a threat. It's hard to feel worried about something you've never seen. The availability heuristic ensures that and it's something that's built into all of us. But if we want to avoid infectious diseases we need at least a little bit of worry. We'll be back with future risk bites to talk more about how feelings influence the way we think about risk. But in the meantime stay safe and feel safe too.