1. Pathogens and their Transmission

Communicable / Infectious Disease Definition

A disease caused by foreign organisms entering the body and multiplying within it

Contagious Disease

One which is passed on by direct contact with the infected person or by contact with someone touched by the person

What is a pathogen?

A disease-causing organism

Bacteria

Most bacteria are harmless to humans (found on skin and in the intestines) but some may cause illness or death

Some damage cells and tissues directly by breaking down the host’s cells for food. Others release toxins that interfere with the host’s normal activity.

Bacteria are:

• single celled

• can be seen under a light microscope

• classified according to cell shape

Diagram of Bacteria

Viruses

• Are too small to be seen with a light microscope

• Contain genetic material either as a molecule of DNA or RNA surrounded by protein coat (some have external lipid envelope)

• Infect a living cell and induces it to manufacture more virus particles which are able to leave that cell and infect other cells

• Bacteriophages - viruses that multiply in bacterial cells

Diagram of Virus

Viral Replication

DNA viruses all replicate in the nucleus, whereas RNA viruses replicate in the cytoplasm

What are the ways of pathogen transmission?

• Physical Contact

• Transfer of Body Fluids

• Infection by Droplets

• Ingestion

• Airborne Transmission

• Transmission by vectors

Physical Contact

Touching an infected person (direct) or touching an object that has been touched by an infected individual (indirect)

• e.g skin infections, STI’s

Transfer of Bodily Fluids

Transfer of pathogens in blood or body fluids from an infected person, through the mucous membranes of an uninfected person or via the bloodstream

• e.g HIV, Hep B, and C

Infection by droplets

Pathogens transmitted through breathing, talking, coughing, or sneezing. Droplets may be inhaled or settle on food utensils and ingested with food.

• e.g. colds, measles, mumps, influenza

Ingestion

Consuming food or drink contaminated with pathogens

• e.g. salmonella food poisoning, typhoid fever, dysentery

Airborne Transmission

Viruses and some bacteria remain viable after evaporation of moisture in exhaled air and can continue to infect when inhaled

• e.g. Tuberculosis (TB) and measles

Transmission by vectors

Other animals such as insects, ticks, or mites transfer the pathogen directly or spread it to food / water which is then ingested

• e.g. malaria and dengue fever (mosquitos), bubonic plague (fleas from rats and mice), and Lyme disease (ticks)