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.