Focal infection theory and its acceptance and discreditation
- Focal infection theory is the historical concept that many chronic diseases are caused by localised infections.
- Focal infections are often asymptomatic and cause disease elsewhere in the host.
- The theory explained virtually all diseases, including arthritis, atherosclerosis, cancer, and mental illnesses.
- Focal infection theory was widely accepted in medicine by the 1920s.
- Focal infection theory was discredited in the 1940s due to research attacks that proved its falsity.
- Dental restorations and endodontic therapy became favored again in mainstream dentistry and medicine.

Alleged foci of infection
- Alleged foci of infection included the appendix, urinary bladder, gall bladder, kidney, liver, prostate, and nasal sinuses.
- Dental decay, infected tonsils, dental restorations, and endodontically treated teeth were blamed as foci of infection.
- Tonsillectomies and tooth extractions were popular approaches to treat or prevent diverse diseases.

Untreated endodontic disease and alternative perspectives
- Untreated endodontic disease retained mainstream recognition as fostering systemic disease.
- Alternative medicine and biological dentistry continued highlighting dental treatments as foci of infection causing chronic and systemic diseases.
- Mainstream recognition of focal infection is endocarditis, if oral bacteria enter the blood and infect the heart.
- Scientific evidence supporting general relevance of focal infections remained slim.
- Evolved understandings of disease mechanisms established a third possible mechanism—metastasis of infection, metastatic toxic injury, and metastatic immunologic injury.

Renewed attention to dental infections
- Dental infections are widespread and significant contributors to systemic diseases.
- Mainstream attention is on ordinary periodontal disease, not on hypotheses of stealth infections via dental treatment.
- Some doubts were renewed in the 1990s by conventional dentistry's critics.
- Dentistry scholars maintain that endodontic therapy can be performed without creating focal infections.
- Dental infections have gained renewed attention in recent years.

Rise and popularity (1890s–1930s)
- Focal infection theory appeared in modern medicine in 1877.
- The breakthrough by Robert Koch in 1882 premised the modern principle of focal infection.
- In 1890, German dentist Willoughby D Miller attributed oral and extraoral diseases to infections.
- Miller identified bacteria in tooth pulp samples in 1894 and advised root canal therapy.
- Ancient and folk concepts found new outlet in medical bacteriology, a pillar of the new scientific medicine.