8.3 - Communicable Disease & Pathogens

Pathogens and the four types

• Most micro-organisms are harmless, and many are useful - they break down dead material and help make foods such as bread, cheese and yoghurt.
• Only a small number, called pathogens, cause disease.
• A pathogen is a microorganism that causes disease.
• The diseases they cause can pass from one host to another, so they're called communicable diseases (also called infectious or transmissible diseases). Both animals and plants can be infected.
• There are four types of pathogen: bacteria, viruses, protists and fungi. Viruses aren't living organisms, but they still count as pathogens.

Bacteria

• Very small living cells - roughly a hundredth of the size of a body cell.
• Once inside the body they reproduce rapidly.
• They make a host feel ill mainly by producing poisons called toxins, which damage cells and tissues.

Viruses

• Not cells at all, and much smaller than bacteria - about a hundredth the size of a bacterium.
• They reproduce by getting inside a living cell and using it to make many copies of themselves. The cell then usually bursts, releasing the new viruses to infect more cells.
• This damage to the host's cells is what makes the host feel ill.
• Structure is very simple: a core of genetic material (either DNA or RNA) surrounded by a protective protein coat, with none of the parts of a living cell.

Protists

• Usually single-celled organisms that vary widely in size.
• The disease-causing ones are often parasites - living on or inside another organism and harming it.
• Many are carried to a new host by a vector - an organism, such as an insect, that transfers the pathogen from one host to another without catching the disease itself.

Fungi

• Can be single-celled, or have a body made of thread-like structures called hyphae.
• The hyphae can grow into and penetrate human skin or the surface of a plant, causing disease.
• Fungi can also produce spores, which spread the infection to other animals and plants.

How pathogens spread

• Pathogens spread from host to host in several general ways.
• The same routes apply to both animals and plants.

Routes of spread

• Air - Carried through the air, often in the tiny droplets released when an infected person coughs or sneezes
• Water and food - Picked up from contaminated water or food
• Direct contact - Passed by touching an infected organism
• Contaminated surfaces - Picked up indirectly by touching a surface or object that carries the pathogen
• Body fluids - Passed through contact with blood and other body fluids
• Animal vectors - Carried from host to host by animals such as insects, which transfer the pathogen without catching the disease themselves