In the United States, 70 percent of all deaths are due to chronic diseases. Until recently, their biological causes were mostly unknown, but today, growing evidence suggests that infections are behind many chronic diseases once thought to be caused by genetic, environmental, or lifestyle factors.
For scientists there are tantalizing clues that a seemingly chronic disease may be infectious.
The human papillomavirus (HPV
), for instance, causes virtually all cases of cervical cancer. It is also responsible for 95 percent of cases of anal cancer, 70 percent of oropharyngeal cancer, and 65 percent of vaginal cancer. The hepatitis B
virus accounts for 80 percent of liver cancer cases worldwide. The hepatitis C
virus causes cirrhosis
, end-stage liver disease, and liver cancer. Human herpesvirus 8 causes Kaposi’s sarcoma, a malignant complication of acquired immunodeficiency syndrome (AIDS). Helicobacter pylori
, a spiral-shaped bacteria, is the agent of peptic ulcers
and gastric cancer. Chikungunya virus infection has been shown to lead to chronic joint pain and arthritic symptoms. Researchers recently discovered a link between Epstein-Barr virus and gastric cancer. It is responsible for about 10 percent of cases diagnosed around the globe. More than 70 percent of all cases are found in the developing world. These examples, however, may be just the tip of the iceberg.
Clues to Infection
For scientists there are tantalizing clues that a seemingly chronic disease may be infectious. When an illness arises mostly in people whose immune system
is weak, it suggests infection (as in Kaposi’s sarcoma following organ transplants). When a disease gets better with antibiotics
(as does strep-induced rheumatic fever), it is likely to be infectious. Another sign of possible infection is chronic inflammation
, which is a common denominator in such diseases as multiple sclerosis, rheumatoid arthritis, lupus, and other autoimmune diseases
. It remains to be proven that any of these diseases have infectious origins, though the possibility certainly exists.
The traditional standards for establishing a microbial cause of disease were developed in the 19th century for acute infections such as tuberculosis
. When it comes to tracking down an infectious source of chronic disease, however, traditional standards may prove to be too restrictive. Sometimes the suspect bacteria or viruses are difficult to detect or grow in the lab. Or there may be long delays between infection and disease, so that by the time symptoms appear, the agents that caused the original infection may be gone. Some psychiatric illnesses, such as schizophrenia
, may have been triggered by infections that occurred just before or after birth. Studies are in progress to address this possibility.
New Treatment Approaches
Proof that certain infections cause chronic diseases raises the promise of treatment with antibiotics or vaccines
. The discovery that infection with H. pylori was associated with peptic ulcers is a well-known example. Doctors used to assume that stress and spicy foods caused ulcers—and recommended bland diets. Today they simply cure the condition by prescribing a pair of antibiotics.
Another advance in prevention is the hepatitis B
vaccine. Liver cancer is one of the most common cancers in the world and the most common cancer in some parts of Asia. With the hepatitis B vaccine now included in universal childhood immunization
programs, new cases of this cancer are expected to drop. The CDC also recommends that preteens and adolescents receive the vaccine for human papilloma virus (HPV), which protects against cervical, vaginal, and vulvar cancers in girls and penile cancer in boys. Unfortunately, only about 40% of boys and 60% of girls are receiving these recommended vaccines. As a result, efforts are under way to encourage parents to have their children protected with this vaccine.