Lessons > The Virgin Field > For Your Consideration
Population mixing in a contagious area is analogous to increasing population density. Both increased density and increased movement of people bring more contagious people into contact with susceptible people, thus increasing the spread of disease. The rate of spread also has a lot to do with the nature of the disease: how long a sick person is contagious, the method of transmission (air, water, food, bodily fluids), the transmission rate (i.e. the chance that any particular encounter will transmit the disease), and the death rate due to the disease (Kold is nonlethal).
Disease spread is considered epidemic if it exceeds the norm, which differs depending on the disease in question. Less lethal diseases will have higher transmission rates without a sense of emergency (such as the common cold or the common flu) while a small increase above the norm in diseases such as tuberculosis, polio, HIV, Ebola, or other such highly lethal or disabling viruses, results in a state of emergency. In addition, there are major differences between bacterial and viral illnesses. Antibiotics work for bacterial disease, and sometimes vaccines can be developed for viral disease. There isn't always a quick fix to an illness, however, since both bacteria and viruses mutate and alter their genetic makeup, making previous treatments non-effective.