Social networks are of huge importance to infectious disease spread. But when I say social network I mean something different from what you may think. When I say social network, you may think of friends and family. Or you may think of online social networks such as Facebook and Twitter, and indeed these kinds of networks do matter too. But really when I say social network I mean a network of people that are connected by potential disease transmission events. Let me clarify. Infectious diseases can cause nasty symptoms and sometimes be a deadly threat. But what makes infectious diseases so dangerous is that they are, well, infectious. The organisms that cause the disease can jump from one person to the next, and thereby setting off a massive cascade of disease transmission. We can conceptualize this by a network, or what's often called a graph. In this kind of network, we think of people simply as nodes in the network, and we put an edge between two nodes only if there is a potential for disease transmission. We can start with any person and connect them to any other person to whom they could potentially transmit the disease. For all of these people, we will do the same. We'll connect them to any other person to whom they could then pass on the disease, and so on and on until we have mapped the entire social network for this particular disease. This social network is like the road system on which a disease can travel. And just like you would want to have a map of all roads when you try to study traffic patterns, we would like to know about the social network when we try to study disease dynamics. It's important to note that each disease has its own social network. That's because the edges in the network represent potential disease transmission events. And that varies from disease to disease. Some networks are probably quite similar to each other. For example, all diseases that are sexually transmitted, probably have a similar social network that's relevant for them. But at the same time if the transmission roots of two diseases are very different then the relevant social networks will be very different. This is why you would not expect any similarity between the networks relevant for something like the flu to something like malaria. Social networks are incredibly important when we think of infectious diseases. A good example is a strategy called contact tracing. Lets say you were studying an outbreak of a disease and you would like to stop the outbreak right away. Hopefully, you've been on the case very early during the outbreak, in which case there would be only a few cases. Contact tracing means to identify all the contacts that a case has had in the recent past. You could follow up with all the contacts and try to protect them and the rest of the population with whatever measures you would find necessary, such as vaccination or quarantining. Indeed, contact tracing was hugely important in recent disease outbreaks such as the Ebola outbreak in West Africa that started in 2014 and the MERS outbreak in South Korea that started in 2015. Contact tracing is essentially a social network strategy starting from a given node, you find all the contacted nodes by following along the edges adjacent to the first node. Then, by vaccinating or quarantining the nodes you found, you are essentially cutting them off from the social network, thereby protecting the rest of the network. We probably won't have very good data on entire social networks for any disease for a long time, if ever. The main reason is that social networks are very hard to measure. Contact tracing is a very resource intensive process, even if you just follow-up on a few cases. Imagine having to do this for thousands or millions of people. That's almost guaranteed to be impractical for various reasons. However, what you can do is map the social networks of some groups like schools, households, workplaces, hospitals, and others. The kinds of groups you'd pick will depend on the disease, and then see if you can find some generalities. Perhaps there are aspects about certain social networks that are very constant across various groups, in which case, it would not be necessary to map the network of an entire population. To summarize, social networks are the road maps for the spread of infectious diseases. Because of that, these social networks are very important to study disease dynamics and to study ways of controlling the spread of disease. At the same time, mapping these networks in great detail is impractical. And so, we need to come up with more efficient ways to understand the structure of these networks. Thankfully, in the past two decades, the emerging science of complex networks has revealed a lot about spreading processes, such as infectious disease spread on a variety of network structures. In another video, we'll look at some of these more common social network structures and how they affect the way in which disease can spread and how it can be best controlled.