Charles Darwin's theories of evolution state that all life on earth developed gradually through natural selection over millions of years. Tracing the branching biological lineage of two organisms backwards, at some point the two organisms would have been related to each other in one way or another. William McNeill argues that, fundamentally, all living things behave the same way and that humans are actually “macroparasites”. Well, we all know that old habits are difficult to change and there should be no doubt that humans and other organisms behave the same way to continue their life and their species. Illness is something that affects all of us, but what really is illness? Many things cause disease. Some diseases can be contracted by drinking contaminated water, others by having unprotected sex, and sometimes by mutations that occur during conception. Disease is basically anything that affects an organism in such a way as to put it at a disadvantage in a certain environment. Mental health problems and alcoholism are both defined as diseases because they both put the person suffering at a disadvantage in their community. Diseases can be seen through various signs and symptoms that occur as a result of the disease. For example, someone addicted to Facebook will exhibit symptoms such as putting off worldly responsibilities or sleeping to chat with friends on the Internet. So, diseases can come in all shapes and forms. Epidemics began to appear for the first time in the period in which man decided to switch from hunting and gathering to breeding and agriculture. The close relationship shared between humans and animals has enabled the transmission of bacteria and viruses from animals to humans. In most cases... middle of paper... no matter how an organism spreads, the mechanism by which it operates is very similar. Biological success is measured through reproduction rather than the age to which something survives. Humans have used disease to their advantage to create a better society for themselves. Biological success says that a successful individual will produce more children, who will in turn produce more children creating a legacy. In this respect, humans are very successful parasites. According to the Merrium-Websters online dictionary, a parasite is “something resembling a biological parasite dependent on something else for existence or support without producing a useful or adequate yield.” Humans have created a way of life by enslaving and living at the expense of other humans and organisms. Ultimately one thing is certain: humans and parasites are much more similar than we would like to admit.