After 9 transmisisons, contagion of 100 people rises to 11,000 people. Mechanisms are the processes which cause one thing to produce another. Contagion: most illnesses that spread from person to person are transmitted by. It is important because it is different from the spread of other things, for every 20 people who got it-18 people got it from them. Exponential growth leads to epidmeic, contagion affects more than non-contagion people. Non-contagion model: 100 infective people, each time about 200 people get infected (linear effect) The types of networks matter as the structure can affect exposure: mode of transmission: colds/flus, cholera (people using same watter means they live near eachother, stds (sexual interaction, needles, blood contact) Study: jefferson high: identified sexual/romantic partners from a roster. Isolation of community is important factor as it is sigicant for the patterns of partnes and sexual partnership choices nothing to do for fun.