Infectious Diseases - Pathogens

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Pathogens cause disease

Pathogens are all around us in the environment every day. Soil contains millions of bacteria per cubic centimetre, fungal spores are in the air that we breathe and microorganism are deposited when pets or people touch surfaces like tables or kitchen work-tops.

How pathogens are spread

There are lots of different ways in which pathogens can be spread to cause disease.

Droplet infection (though the nose)	When you cough, sneeze or talk tiny droplets fly out of your mouth and nose. If you have an infection, those droplets will contain microorganisms. Other people breathe in the droplets, along with the viruses or bacteria they contain. Diseases such as tuberculosis and the common cold are spread in this way	Photo courtesy of CDC
Direct contact (eg reproductive organs)	Some diseases are spread by direct contact of the skin and some sexually transmitted diseases like genital herpes
Contaminated food (through the mouth)
Contaminated water (through the mouth)	Drinking water contaminated by sewage means you take large numbers of microorganisms straight into your gut eg cholera and amoebic dysentery. Anywhere without proper sewage treatment and disposal is at risk of these diseases.
By body fluids (through breaks in the skin)		Photo courtesy of AVERT
Vectors (usually through the skin or through the mouth)		Photo courtesy of CDC/Dr William Collins

In shanty towns and slums around the world, and wherever there is a natural disaster, you find many of the conditions which lead to the easy spread of disease.

People are displaced from their homes and are crowded together so droplet infection happens easily.
Damaged sewage systems and water supplies mean the water is often contaminated with sewage.
Damaged electrical supplies mean fridges don't work, so food goes off quickly and flies and other insects often contaminate the food.
Malaria mosquitoes may breed in standing water.
Health services are disrupted so diseases cannot be treated quickly so they spread fast.

Natural disasters mean many different ways of spreading pathogens all come together to cause a major health risk.
This may cause more deaths than the original disaster
Courtesy of Julien Harneis

Antibiotic

Medicine that acts against bacterial infections. Penicillin is an example of an antibiotic.

Antibody

Protein that is produced by lymphocytes (white blood cells) and that attaches to a specific antigen.

Antigen

Molecule on the surface of a pathogen that identifies it as a foreign invader to the immune system.

Bacteria

Single-celled organism. Has a cell wall, cell membrane, cytoplasm. Its DNA is loosely-coiled in the cytoplasm and there is no distinct nucleus.

Biotechnology

The use of biological organisms or enzymes to create, break down or transform a material

Dissect

To cut apart, or separate, tissue especially for anatomical study.

Exponential growth

If something is growing exponentially the larger the quantity gets, the faster it grows

Fungus

Micro-organism that can grow in long tubes called hyphae or as single cells. Fungi have a nucleus, cytoplasm and a cell wall.

Herd immunity

If a high percentage of a population is immune to a disease the disease cannot be passed on because it cannot find new hosts.

HIV/AIDS

Infection caused by the human immunodeficiency virus (HIV). It attacks and destroys the immune system.

Hybridoma

Hybridoma cells are formed by fusing a specific antibody-producing cell with a type of cancer cell that grows well in tissue culture

Immune system

The body's natural defence mechanism against infectious diseases.

Immunisation

A process which gives immune resistance to a particular disease. The human or animal is exposed to a harmless antigen in order to raise antibodies and provide an immune memory.

Lymphocyte

A type of white blood cell that make antibodies to fight off infections.

Macrophage

A type of white blood cell that consumes dead pathogens that have been killed by antibodies.

Parasite

Organism that feeds off another living host and causes it some damage. An example of a parasite is a tapeworm that lives in the digestive system of a host organism.

Pathogen

A micro-organism that causes disease.

Phagocyte

Phagocytes are the white blood cells that protect the body by ingesting harmful foreign particles, bacteria, and dead or dying cells.

Protein

A polymer made up of amino acids joined by peptide bonds. The amino acids present and the order in which they occur vary from one protein to another.

Protozoa

Protozoa are one-celled animals

Spore

A spore is a reproductive structure that is adapted for dispersal and surviving for extended periods of time in unfavourable conditions.

Toxin

A poisonous or toxic substance - produced by pathogens.

Vaccination

A small amount of dead or weakened pathogen is introduced into the body. It prepares the immune system to prevent future infections with the live pathogen.

Vaccine

Medicine that contains a dead or weakened pathogen. It stimulates the immune system so that the vaccinated person has an immunity against that particular disease.

Virus

The smallest of living organisms. Viruses are made up of a ball of protein that contains a small amount of the virus DNA. They can only reproduce after they have infected a host cell.

Opportunistic Infections

An infection that would not normally cause disease in a healthy person but which can take hold when a person's normal immune defences are reduced.