An organism is considered to have an infection when a disease-causing agent, called a pathogen, establishes a viable presence in the organism. This can occur only if the pathogenic agent is able to reproduce itself in the host organism. The only agents believed until recently to be responsible for infections—viruses, bacteria, fungi, and parasites—reproduce and regulate their other life processes by means of genetic material, composed of nucleic acid (DNA or RNA). It was thus widely assumed that all pathogens contain such genetic material in their cellular structure.

This assumption has been challenged, however, by scientists seeking to identify the pathogen that causes Creutzfeldt-Jakob disease (CJD), a degenerative form of dementia in humans. CJD causes the brain to become riddled with tiny holes, like a sponge (evidence of extensive nerve cell death). Its symptoms include impaired muscle control, loss of mental acuity, memory loss, and chronic insomnia. Extensive experiments aimed at identifying the pathogen responsible for CJD have led surprisingly to the isolation of a disease agent lacking nucleic acid and consisting mainly, if not exclusively, of protein. Researchers coined the term "prion" for this new type of protein pathogen.

Upon further study, scientists discovered that prions normally exist as harmless cellular proteins in many of the body's tissues, including white blood cells and nerve cells in the brain; however, they possess the capability of converting their structures into a dangerous abnormal shape. Prions exhibiting this abnormal conformation were found to have infectious properties and the ability to reproduce themselves in an unexpected way, by initiating a chain reaction that induces normally shaped prions to transform themselves on contact, one after another, into the abnormal, pathogenic conformation. This cascade of transformations produces a plaque, consisting of thread-like structures, that collects in the brain and ultimately destroys nerve cells. Because prions, unlike other pathogens, occur naturally in the body as proteins, the body does not produce an immune response when they are present. And in the absence of any effective therapy for preventing the cascade process by which affected prions reproduce themselves, CJD is inevitably fatal, though there are wide variations in pre-symptomatic incubation times and in how aggressively the disease progresses.

Although the discovery of the link between prions and CJD was initially received with great skepticism in the scientific community, subsequent research has supported the conclusion that prions are an entirely new class of infectious pathogens. Furthermore, it is now believed that a similar process of protein malformation may be involved in other, more common degenerative neurological conditions such as Alzheimer's disease and Parkinson's disease. This possibility has yet to be fully explored, however, and the exact mechanisms by which prions reproduce themselves and cause cellular destruction have yet to be completely understood.