For scientists there are tantalizing clues that a seemingly chronic disease may be infectious. When an illness arises mostly in people whose immune systems are weak, it suggests infection (such as in Kaposi’s sarcoma following organ transplants). When a disease gets better with antibiotics (as does strep-induced rheumatic fever), it’s 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 and anthrax. 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.
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.
As even this brief survey of disease threats shows, trends in infectious disease are never stagnant. They reflect dynamic forces in the world, some of which are the subject of the next section. The microbe–human relationship continues to evolve in ways that may not always be predictable, especially as we exert ever-greater stresses on the planet in our endless quest to survive and prosper.