Indirect transmission requires that the disease-causing microbe come in contact with some intermediate object or substance before the microbe contacts the second host. Non-living objects that may transmit microbes are called fomites. The list of possible fomites is limitless, but some common examples include bedding, toilet seats, syringes and needles, cups and other eating utensils, school toys, clothing, and other items in public facilities. Because paper money is impregnated with a disinfectant and door knobs and public phones are often sanitized, their incidence of microbe transmission is probably less than you might think.

Contact with infected animal products may result in zoonotic diseases. Zoonosis are diseases of animals that can be transmitted to humans. A couple examples of this type of transmission include anthrax transmitted by infected wool or hides. The Bacillus organism is responsible for anthrax or woolsorter’s disease. Unpasteurized or raw mild from cows infected with Brucella may transfer the microbes to humans resulting in human illnesses similar to brucellosis.

Food infected with microbes is an important indirect method of transmitting microbes. The oral-fecal route is a method of contaminating food as is food contact with soil and human flora. Hepatitis A, Salmonella, Clostridium, Staphylococcus, and Mycobacterium are just a few microbes transmitted by food; and the subsequent diseases could be typhoid fever, botulism, and tuberculosis.

Water that has been contaminated with pathogenic microbes can transmit the disease indirectly. Feces or urine permitted to come in contact with drinking water may transfer microbes responsible for cryptosporidiosis, polio, or many other diseases.

When mucus and saliva that contain microbes are coughed or sneezed in to the air, it is possible that dried residue called droplet nuclei may form. These droplet nuclei may remain infective for a relatively long time. Gram positive bacteria like Streptococcus and Staphylococcus may survive 2-3 months in dry dust. The measles virus and Mycobacterium (which causes tuberculosis) also survive in the air, but Mycobacterium is more sensitive to sunlight so many TB sanitariums have sun porches to help reduce the microbes involved.

If you are working with organisms with Q-fever, birds with psittacosis (parrot fever), dry mopping floors, or making patients’ beds, you must take the necessary precautions to prevent the intaking (inhaling) tiny aerosol particles. In addition, burn victims and patients on immuno-suppressive drugs should be kept in rooms with positive air pressure so the airborne microbes will move out the door. One bacterium known to survive and be transmitted through ventilation is Legionella, the microbe that causes legionnaire’s disease.