The Nature of Infectiousness


What makes something infectious?

Some diseases are contagious - they spread by contact, Infectious diseases spread by infectious agents - bacteria and viruses for instance. Contagious diseases are infectious, but not all infectious diseases are contagious.

Infectious agents are transmitted from person to person in any of a hundred ways: When a sick person sneezes, tiny particles of water are sprayed into the air, where some may enter other people through the mouth or nose. Liquids are effective vectors of disease because microorganisms can live in them more readily that on dry surfaces (generally speaking). Infectious medical waste is often (not always) wet. The job of the waste manager is to disinfect the waste before final disposal and to keep people from becoming infected from the waste in the meantime.

Viruses, bacteria, and spores

Microorganisms that cause disease are called pathogens. Most microorganisms are not pathogens; they are benign. Bacteria are single-cell organisms. The cells are similar in many ways to the cells that make up our bodies. Some bacteria cause disease. Bacteria are susceptible to antibiotics which kill them or stop them from reproducing, and antibiotic materials are widely used in disinfection and sterilization processes. Viruses are clumps of genetic material surrounded by a protein coat. They are not truly alive in that they don’t eat or use energy. Once viruses get inside other cells, they can hijack the cellular processes and cause replication of themselves. This causes disease. Viruses are not susceptible to antibiotics, but can be denatured or deactivated by high temperatures or harsh chemicals.

Mold and mildew are both fungi. They are usually not harmful or toxic to people. (A few forms of mold are dangerous.) Fungi produce spores to reproduce. The spores have thick walls and are hardier than most fungi cells. When people want to eliminate fungi they concentrate on killing spores.

Prions are infectious proteins; theu strike fear into the hearts of infection control officers. They cause lethal brain diseases, and cannot be eradicated by washing, most chemical disinfectants, or boiling water. Scientific American magazine says prions should be in the “Hard-to-Kill Hall of Fame” Like viruses they are not really alive. Much is unknown about how they cause disease. If you suspect your waste has prions in it, call in an expert. There is evidence that alkaline hydrolysis processes used on animal carcasses eradicate them, but much is not clear.

Some bacteria also produce spores called endospores. These allow the bacterial population to survive changes in the environment - to wait out harsh conditions before coming back to live as regular bacteria.

Before prions came to the limelight, endospores were considered the hardest pathogens to destroy. While heating a material to 150 degrees F for 20 minutes will kill most bacteria and viruses present, endospores can survive. This is why autoclaves operate at temperatures of 250 degrees F and higher.

Industrial hygienists and public health pros generally consider these pathogens the hardest to kill, in order from hardest to easiest:

  1. bacterial spores
  2. mycobacteria
  3. hydrophilic viruses
  4. lipophilic viruses
  5. vegetative fungi and fungal spores
  6. vegetative bacteria - also called "natural flora"

Prions are the big unknown here, patially because they have been discovered more recently, methods to detect them are not widespread, and it is not clear which diseases are caused by prions. However, experts think they should be at the top of the list (harder to destroy than bacterial spores) or close to the top.

biohazard synbol Quick: if you boil a pan of water on your stove and put used surgical equipment in the water, will it sterilize the equipment? Answer: officially no. As a practical matter heating the equipment will probably remove pathogens, but some spores can withstand boiling water which is why waste management practitioners heat equipment to higher temperatures. A 250 degree Fahrenheist (= 120 degree Celcius) temperature is the standard sterilization temperature.