Infections are around us all our lives. Most seem to be short-lived, miserable bouts of intestinal or respiratory bugs that so often drive us to a doctor.
But researchers have been steadily peeling away the impact that germs and viruses can have in producing other medical problems.
The human papillomavirus causes almost all cases of cervical cancer. Hepatatis B virus is blamed for more than 60 percent of liver cancers. A chronic bacterial infection causes ulcers, and there's continuing research pointing to inflammation from infection contributing to the formation of plaque that brings about heart disease.
Many other connections are suspected between infectious agents and conditions affecting the immune system, from multiple sclerosis and arthritis to diabetes and obesity.
Recently, researchers have found evidence that infections can affect humans even before birth. While the link between sexually-transmitted infections such as Chlamydia and gonorrhea and infertility has long been established, the ties to other types of infection are only beginning to be understood.
One study, presented in July during a European meeting on reproduction, reported that chronic gum disease in women can have a significant impact on their ability to conceive, on par with obesity as an obstacle.
The Australian scientists tracked conception and pregnancy among more than 3,400 women. They found that women with gum disease took an average of seven months to become pregnant, two months longer than women with good oral health.
The effect was even more pronounced among non-Caucasian women. Their increased risk of later conception was nearly 14 percent compared to just over 6 percent for those without gum disease.
Roger Hart, a professor at the University of Western Australia who led the study, said the effect held true even taking into account other negatives for conception such as smoking, being overweight or 35 years of age or older.
"Gum disease is a risk factor that can be modified and indicates that a dental visit should be part of a prenatal checkup of health," Hart said, adding that non-white women may be more affected because they tend to have a higher level of inflammatory response to the infection."
The study was part of a larger research effort that considered whether treatment of gum disease helps prevent premature birth. Those results suggested such care does not prevent pre-term birth in any ethnic group, but neither does it cause mother or infant any harm.
That confirms findings from several other studies done in recent years.
However, scientists are still looking at possible ways to prevent gum disease in women of child-bearing age as a way of reducing premature birth rates.
For instance, a researcher at Baylor College of Medicine recently won seed funding from an international collaborative called Saving Lives at Birth to study whether the sugar alcohol Xylitol used to flavor chewing gum, mints and candies can help prevent gum disease and reduce early birth.
The idea is to supply large number of women in Malawi with the gum and mints over a period of time and measure the impact on pregnancy and delivery in various regions around the country.
"Preterm birth is a tremendous maternal and newborn health problem in Malawi," said Dr. Kjersti Aaagaard. "Rates of preterm birth there are more than double that in the United States and nearly triple that of the developed world," she added. "The overwhelming majority of these babies will not survive due to the lack of neonatal care resources."