They go just about every dirty and dark corner of the planet. Through their nocturnal excursions they spread disease.
According to the National Pest Management Association, there are approximately 33 different kinds of bacteria, six kinds of parasitic worms, and at least seven other kinds of human pathogens carried by cockroaches. Consider the fact that cockroaches walk through sewage and decaying matter. They transmit the bacteria into your home. In addition, roaches can be an allergen for allergy sufferers.
They have been found to carry the pathogens that cause tuberculosis, cholera, dysentery, and typhoid; as well as 28 other bacteria (like salmonella). Because of they way they live, they can carry just about any bacteria or disease that lives on decaying organic matter or sewage.
With salmonella, you will get diarrhea, abdominal cramping, fever, and nausea. It is most often found in animal products like beef, poultry, milk, or eggs. Once way to avoid salmonella is to avoid cross contamination. For example, many products with salmonella are thoroughly cooked and the person does not get infected.
What roaches do is walk all over uncooked decaying matter. In essence, they spread the germs around. If salmonella gets into your home, the roaches can transmit it onto areas that would not otherwise have it.
Roaches love dark damp places. What grows in these damp places is mold. Many roaches actually die from mold. What happens is the roach gets real moldy and then sheds the exoskeleton. As the exoskeleton decays, mold and roach exoskeleton irritate allergy sufferers. Some people say the roach is second only to the dust mite in allergy irritation.
Boric acid roach bait is a fantastic solution. We call it MRF 2000.