SciShow is supported by Brilliant.org. { ♪INTRO }.

The word “contagious” probably conjures up all sorts of horrific images of Ebola or the plague— diseases that have a reputation for gruesome, indiscriminate killing. So you might be surprised to hear that when it comes to the world's most contagious diseases, those don't even make the top 10.

The winners on that list can be ten or more times as contagious as Ebola or the plague, and they aren't off in some remote jungle or a thing of the past, either. They're right here, right now. In fact, measles—that almost-eliminated virus now making headlines in places like Washington and New York— is one of the most contagious diseases we know of.

And it's contagiousness is a big part of why it's making a resurgence. Now, the word “contagious” has a very specific meaning to epidemiologists —the scientists that study how diseases spread. A disease is considered especially contagious if one sick person is likely to infect a lot of other people.

And that's something that can be quantified. The average number of people a single sick person infects is known as a disease's basic reproduction number or R0. R0 is based on a number of factors, like the rate of contact between infected and susceptible people how long an infected person is contagious,.

Probability that an uninfected person will contract the disease if they're around someone who's sick. Because of that, it's not a fixed number— there are a lot of variables that can impact how contagious a pathogen will be in a specific environment. So contagiousness can vary from location to location and outbreak to outbreak.

When R0 is less than 1, a disease is considered self limiting. Basically it's not going to spread through a population, and will eventually just die out. When R0 is 1, a disease will kind of hang around, but it won't cause an outbreak.

Above 1, and things start to look bad because the number of infected people increases exponentially. To put that in perspective, the most contagious diseases we know of have double or even triple digit R0s. That's why a disease like ebola—which has an R0 between 1.5 and 2.5—can cause epidemics but isn't considered super contagious.

It is very infectious, though. Infectiousness is measured by the infective dose— basically, how many virus particles, spores, bacterial cells, or other sadistic little vehicles of doom it actually takes to make you sick. Ebola is also really deadly.

So basically, it's extra-super terrifying. But if someone in your town has ebola, you could probably hit up the local grocery store without worrying— that's not true for really contagious diseases like measles. Measles is a vaccine-preventable disease that was basically eliminated from the United.

States at the turn of this century, but now it's clawing its way back. Yay. Measles R0s from previous outbreaks vary a lot, from about 5 to several hundred depending on factors like population density and vaccination status.

But taken together, it's clear that if no one is vaccinated, then an infected person with measles will likely infect more than a dozen other people, each of whom can go on to infect another dozen or more people, and so on. And it takes vaccination rates above 95% to push that R0 to 1 or less to keep it in check. One of the main reasons measles is so contagious is that, kind of like ebola, it's really infectious—it doesn't take a lot of viruses to get you sick.

If you are in close contact with someone who has measles and you're unvaccinated, your chances of getting sick are about 90 percent. Meanwhile, during flu season, you probably breathe in influenza viruses any time you enter an enclosed, public space like an airplane or a daycare without getting sick… most of the time. Another big reason it gets around is its route of transmission.

Measles can go airborne, where it can linger for up to two hours. Theoretically, someone with measles can walk into an elevator, cough, and then two hours later you can walk into the same elevator and catch it. You don't even have to be in the same room at the same time!

And measles has a way to ensure a lot of little infectious viruses get into the air with every cough. As they say in real estate, it's all about location, location, location. The measles virus makes itself at home in your trachea, at the top of your respiratory system, so it's in a prime position to come out when you cough or sneeze.

Other respiratory viruses, like influenza, have to travel much further in order to get out of your body. And Ebola, by comparison, is spread through bodily fluids. So you have to come into contact with things like blood, vomit, or feces in order to become infected.

Which is much less likely than the elevator scenario, though infinitely more disgusting. Also, people with ebola aren't contagious until they develop symptoms. So they're not really likely to wander around during the contagious period when they can spread the infection.

People with measles can be contagious up to four days before the characteristic rash develops. So they might think they have a cold, shrug it off, and spend days walk around coughing, sneezing, pushing elevator buttons, shaking hands, or going to Disneyland, before it even occurs to them they should stay home. This sort of thing happens with other diseases, too—like the bacterial illness pertussis, also known as whooping cough.

Pertussis generally has an R0 of 12 to 17, which probably has something to do with the fact that adolescents and adults often get a mild, cold-like version of the disease, or have no symptoms at all. That's simply not the case with influenza, which has a low R0 of around 1 or 2. People with the flu are only contagious for about a day before they develop symptoms.

And they're most contagious when they're three or four days in—when they're most likely in bed. One of the only viruses to top measles' claim to contagious fame is rotavirus, the nasty stomach bug that's most common in children. It has an average R0 of about 25.

And that's probably because, like measles, it's extremely infectious, but it's even hardier. It causes explosive diarrhea and is most common in children, so, well, you can extrapolate from there how it gets around. Also, the virus is present in an infected person's feces before they develop symptoms, and it can last hours on unwashed hands, days on some surfaces, and weeks in drinking water.

In fact, small kids with questionable hygiene is a common theme when it comes to super-contagious diseases. That certainly helps measles move around schools and daycares, and it's part of why chickenpox generally has an R0 of 10 to 12. The virus can be passed through the air, but it's also present inside the pustules that develop, so it can get all over kids' hands when they scratch those itchy lumps.

The virus that causes chickenpox probably doesn't survive too long outside the human body, though—which is why it doesn't quite have the legs that measles does. So, in conclusion, you should stockpile beans, rice, and bottled water and never leave your house. Or, you could take reasonable precautions against contagious diseases.

The contagiousness of viruses like measles is a big part of why we've developed vaccines against them—because even if they're not as deadly as ebola, they are still deadly. The likelihood someone will die depends a lot on where they live — in developed countries like the United States it's around around a half a percent, but in developing countries it's closer to 6%. Unfortunately, some people, like young babies or people with compromised immune systems, aren't able to get those vaccines.

So if you want to help protect everyone around you and stay safe yourself, you can get vaccinated and get your children vaccinated on the recommended schedule. And don't forget to do the usual common sense things like wash your hands frequently, and, for the most part, steer clear of other people's bodily fluids. I mean, you might luck out and stay healthy no matter what.

But taking such simple steps can reduce your odds of getting sick. And if you want to really understand what it means to reduce those odds, you might want to check out the Probability course on Brilliant.org. Probabilities are all around us all the time, and this course helps you get to know them inside and out by explaining the math that underlies things like poker strategy and weather prediction.

That, in turn, can help you make more informed decisions, from the best bet to make with your cards to whether you should carry your umbrella today. It's one of the many interactive lessons and quizzes in math and science Brilliant offers. And right now, the first 200 people to sign up at Brilliant.org/SciShow will get 20% off of an annual premium subscription to Brilliant.

So you can learn a lot, have fun and know that you're supporting SciShow too! { ♪OUTRO }.