From gallstones to arthritis to heart disease, many illnesses long presumed to have roots in genes or lifestyle may be caused largely by infectious agents, a growing number of scientists believe. Most of the evidence, however, remains circumstantial.
Evidence is mounting that a variety of common germs long thought to cause only mild, short-lived illnesses such as the flu play a role in causing chronic health problems ranging from allergies, asthma
to obesity, heart disease and cancer. Long after one recovers from the microbe's initial insult, viruses, bacteria
and other germs silently chew away at the body's tissues and organs, causing insidious, permanent damage, it is believed.
For some conditions, many scientists feel certain that microbes play at least a contributing role where none was suspected previously. A microbe may be suspiciously present in people who have a disease, for example, and not in those who don't, suggesting causality.
Microbe hunters now estimate that anywhere from one-third to more than one half of chronic diseases will eventually be explained by infection with a variety of microorganisms. They point to new research that links germs to some forms of infertility, kidney
, stomach problems and even obsessive-compulsive disorder
No one can avoid the wide variety of microorganisms that are in our environment. Within minutes of birth and continuing throughout our lives, our bodies are exposed to virus after virus, bacterium
after bacterium. Under normal circumstances, the body's natural defenses fight off harmful microbes, but when our immune systems are weakened, when our normal defense barriers are not intact, or when we come in contact with a highly invasive infectious agent that is either new to our bodies or present in large numbers, the balance shifts in favor of the microbes.
Modern living has helped to tip the scales in favor of the tiny, microscopic bugs: international travel, misuse and overuse of antibiotics, and air pollution, to name but a few, all improve the efficiency by which germs spread from person to person and place to place.
Causes and Development
Exactly how germs act to cause chronic disease is still a mystery. But scientists believe that the microbes may stimulate white blood cells
to aggregate, causing the chronic inflammation
that has now been implicated in asthma
, allergies, heart disease and other disorders. Alternatively, the bug may induce a so-called autoimmune
response. There are molecules on the surface of bacteria
and viruses that resemble human versions of these molecules, and when infected with these organisms, certain people respond by producing substances that attack their own tissue.
In trying to fight off an infection, in other words, your body's defense cells are tricked into attacking the healthy tissue. Among the microbe-disease links now suspected or proven: Ulcers
. After years of being shunned, Dr. Barry Marshall's theory that ulcers can be caused by the bacteria Helicobacter pylori
is now accepted medical doctrine. The link between infectious diseases and cancer is becoming increasingly clear: According to the World Health Report, up to 84% percent of certain cancers, notably, stomach, cervical, and liver
, are attributable to a variety of germs.
The implications of this new theory are enormous, researchers say. Most important, it suggests that vaccines, anti-viral drugs or antibiotics may have an unexpectedly big role to play in the treatment of chronic diseases that today are treated with only modest success through lifestyle changes, such as exercise and improved diet. There is also likely to be a role for anti-oxidant vitamins, which can remove some of the damaging molecules that the microbes leave in their wake.
It wasn't easy getting the medical community to accept such a paradigm shift in thinking. But new testing methods that allow molecular biologists to detect the footprints of the microscopic creatures long after any symptoms disappear are slowly winning over even the most recalcitrant.
"If an infectious agent is responsible for even a portion of these diseases, that could change the outlook for treatment and prevention dramatically," said Barry Bloom, dean of the Harvard School of Public Health.
For example, a so-called adenovirus, the same type of germ that causes the common cold, may be to blame for the excess pounds you can't seem to shed, according to Nikhil Dhurandhar of the University of Wisconsin in Madison. In a study of 154 obese people, 15% had evidence of infection with an adenovirus called Ad-36.
Baltimore scientists reported in 2002 that they have found the first hard evidence that viral infections can cause asthma
and allergies in humans, suggesting that vaccinations against the viruses could prevent the diseases.
In test-tube studies of human cells, the researchers showed that weak viral infections can cause immune system cells called B cells to produce immunoglobin E or IgE
, a protein that orchestrates the reactions that cause allergies and many cases of asthma. Still other studies have implicated mycoplasmas, germs of intermediate size between viruses and bacteria
, and the lung
bacteria pneumoniae in asthma.
Evidence is mounting that coronary heart disease may be caused in part by inflammation
that silently simmers away for years inside the blood vessels, and that chronic infection with common bacteria or viruses may play a role in causing the chronic inflammation in the first place. Among the suspected culprits: the ulcer-causing bacteria H. pylori
virus, or even chronic tooth decay.
The common respiratory bug Chlamydia pneumoniae
has been linked in new studies to arteriosclerosis
, or hardening of the arteries
. And still other studies have shown that regrowth of plaque in patients who have undergone surgery to open up clogged arteries may be spurred by cytomegalovirus
a bug found in almost two out of three elderly Americans.
New evidence suggests that both the common respiratory bug Chlamydia pneumoniae, as well as another type of germ called mycoplasma, can cause arthritis
. The findings may explain why so many arthritis patients get relief from joint pain and swelling after antibiotic treatment.
Researchers have found that mycoplasmas can live in a woman's reproductive tract, causing infertility, premature birth and spontaneous abortion