There are numerous ailments that can afflict the human body, ranging from carsickness to colds to cancer. The earliest physicians thought that illness and disease were a sign of God's anger or the work of evil spirits. Hippocrates and Galen advanced the concept of humorism, a theory which held that we get sick from imbalances of the four basic substances within the human body, which they identified as blood, phlegm, black bile and yellow bile. Paracelsus, a Renaissance-era physician, was one of the first to posit that sickness comes from outside sources, rather than from within.
Today, we know that there are two major kinds of diseases: infectious and non-infectious. Infectious diseases are caused by pathogens such as bacteria, viruses, fungi and parasites. These pathogens can enter the body through the air we breathe, the food and drink we consume or through openings in the skin, such as cuts. As an example, think of a person who has a cold. That person may cough into his or her hand and then touch a doorknob, thus placing the cold virus on that doorknob. The virus may die on the doorknob, but it's also possible that the next person to touch the doorknob will pick it up. If that person then touches food with the unwashed hand and consumes the food, the virus is now inside the body.
Not every pathogen that enters the body results in illness -- our bodies come equipped with immune systems to fight off foreign agents. However, pathogens have the ability to adapt and evolve much more quickly than the immune system can, which means that pathogens sometimes have the upper hand when it comes to fooling the body's defenses. One way that pathogens evade the immune system is by hiding within the body's healthy cells. Additionally, some people have weakened immune systems that make it harder for them to resist the effects of an invading pathogen.
Non-infectious diseases aren't caused by pathogens and can't be spread person-to-person. These diseases are more likely to be caused by a confluence of factors including the environment, a person's lifestyle choices and genetics. For example, skin cancer is usually the result of people spending too much time in the sun without protection from the sun's UV rays, which is considered an environmental factor. A condition like heart disease may be caused by a sedentary lifestyle and a poor diet, or it may be caused by a family history of the disease. Though we may not be able to change our genetic code, there are plenty of things that humans can do to prevent noninfectious diseases. Most notably, we can choose to eat healthfully and exercise. We can also reduce our exposure to avoidable risk factors such as cigarette smoke.