Germ theory of disease
The Germ Theory of Disease (also called the pathogenic theory of medicine) is a scientific theory that suggests that microorganisms are the cause of many diseases. This theory was extremely controversial when first proposed, but today it is the backbone of modern medicine.
Prior to germ theory, it was widely believed that disease was spread by smell. As disease-spreading conditions - rotting meat, open sewage, and such things - was often highly odorous, the smell and disease were associated. Disease was also often attributed to demonic influence - although demonic posession must exist as it it mentioned in Scripture[1], it is a rare occurance, and almost all disease attributed to it was a result of other causes.
The Christian Scientist sect, founded by Mary Baker Eddy, believes that the Germ Theory of Disease is not true and that it is contradicted by Scripture.[2] God created all life, and so all life should reflect His glory and inherent goodness - God could not have created pathogenic organisms, as these are inherently contary to His nature.