The host is the second component of the epidemiologic triangle. For disease transmission, perhaps the most important attribute of a host is their age. Age affects almost every aspect of infection: the likelihood of becoming infected, the severity of disease if infected, and the likelihood of transmitting it to others. An individual's age affects the types and frequencies of social interactions, such as attendance at group child care, school, and work. It also affects the tendency to engage in activities that can transmit specific types of infection, such as sexually transmitted infections. The frequency and intensity of these interactions affect the risk of being exposed to an infectious person and the rate of passing on infections to other susceptible persons. For many infectious diseases, infection leads to immunity. This immunity may protect the individual against re-infection for many years, or even for life. Typically, for such diseases, children are at greatest risk of contracting the infection because they are less likely to have had it before. However, in the first half year of life there may be some protection against infection from antibodies the child receives from the mother transplacentally before birth and in breast milk. For some infections, such as influenza, pertussis, and tetanus, we use this fact as a public health strategy and offer vaccines to pregnant women to protect their children after birth. In many diseases the immune response begins to decline with advancing age, resulting in a distribution of disease incidence that has two peaks: one after the decline of maternal antibodies and another in old age. More generally, the health of an individual's immune system can affect their susceptibility to infection and disease. Diabetics, people undergoing certain types of cancer treatments, and other immuno-compromised people are at elevated risk of contracting certain infections. Some infections predispose a host to acquire other infections. Perhaps the best known example is HIV. HIV increases a person's risk of many diseases: including bacterial pneumonia, tuberculosis, and some other infections. Some so-called “opportunistic infections” are common in HIV patients but are almost never observed in people with healthy immune systems. In fact, the discovery of the HIV virus began with an observation of unusual fungal pneumonia and an unusual, infectious form of cancer, known as Kaposi's sarcoma, which were seen in men who have sex with men. Epidemiologists were able to work out that these men had compromised immune systems, which make them susceptible to the unusual infections that first brought them to medical attention. HIV was discovered as the cause of this immuno-compromised state. Other examples in which one infection increases the risk of another infection include influenza, a viral disease which can lead to secondary bacterial pneumonia, and schistosomiasis, a helminth or worm disease that can increase one's risk of acquiring HIV. A person’s behavior affects their risks of becoming infected and passing on infection to others. One type of behavior that is extremely variable across different hosts is the level of sexual activity, which may range from abstinence or long-term monogamy for many individuals, to extremely high rates of new partner acquisition for others. This variation has important consequences both for the maintenance of sexually-transmitted diseases in populations and for their control. Sometimes particular hosts are unusually infectious to others because of the site at which they are infected. Tuberculosis infection in the larynx or throat is especially likely to transmit to others, while extrapulmonary TB or infections outside the respiratory tract are much less likely to be transmitted. Finally, sometimes people vary in the number of secondary cases they infect, and we don't know the reason. During the SARS epidemics in 2003, there were a number of super spreading events. Whereas a typical SARS case infected about three other people in the period before control measures were introduced, there were a few instances in which so called “super spreaders” infected upwards of 20 other individuals. In this session we have had an overview of the major host factors that affect transmission and severity of disease. In the upcoming sessions we will focus on two of these in more detail.