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Human immune system
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Pathogens and disease

Pathogens are microbes that cause disease. They are constantly all around us in the environment and have evolved to spread from one person to the next in a variety of ways. Four types of pathogens cause problems in humans. Click on the pathogens below to find out more.
See Infectious diseases and their treatment for more information.

Transmission of infection

Pathogens need to be able to spread from one person to the next so that they can reproduce and find new sources of food. This can happen in many ways.

Try the question below to find out more about transmission.
Question 1

The table below shows six diseases. For each disease, choose the method of its transmission and the best way of preventing transmission. You should make 12 choices in total.

   HIV SARS Malaria Amoebic
dysentry		 Tetanus Salmonella
Method of transmission
In tiny droplets from coughs and sneezes.
By eating contaminated food.
During sexual intercourse and by drug-users sharing needles to inject drugs.
From bacteria getting into a cut or wound.
By drinking infected water.
In the bite of an infected mosquito.
Prevention
Sleeping under insect-proof nets impregnated with insecticide.
Covering mouth or using handkerchief.
Keep cuts clean and covered. Also vaccine available.
Correct storage and preparation of food.
Using a condom. Drug users do not share needles.
Keeping drinking water separate from water used for bathing and sewage.
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Bacteria

Description: 
Single-celled organisms that have a cell wall, cell membrane and cytoplasm. DNA is present as a loosely-coiled chromosome-loop.
 

Size: 0.01 mm
 

Reproduction:
Most reproduce asexually by copying cell structures and then splitting into two new bacteria. The process is called binary fission.
In favourable conditions of pH, temperature and nutrients, millions of bacteria can quickly grow from a single contaminating source.
 

Diseases include:
Throat infections - Streptococcus pyogenes.
Tuberculosis - Mycobacterium tuberculosis.
Food poisoning - Salmonella typhimurium, E. Coli.
Boils - Staphylococcus aureus.
Cholera - Vibrio cholerae.
 
Fungi

Description: 
Can grow as either individual microscopic cells or large strands of thread-like strands called hyphae.
Fungi have a cell membrane, cell wall, cytoplasm containing mitochondria and a visible nucleus.
 

Size: 0.01 mm to several cm when growing as a mesh of fibres.
 

Reproduction:
Fungi can grow very rapidly by asexual reproduction. They also form specialised structures that are involved in sexual reproduction with other fungi of the same species.
Mushrooms and toadstools are visible reproductive organs of fungi.
 

Diseases include:
Thrush - Candida albicans.
Ringworm - Trichophyton mentagrophytes.
Athlete's foot - Tinea pedis.
Aspergillosis (lung infection) - Aspergillus
 
Viruses

Description: 
Much smaller than a cell, a virus is just a sphere of protein containing the virus genetic material. This can be in the form of DNA, or another type of nucleic acid called RNA.
Viruses have only a few genes and so no chromosomes or a nucleus.
 

Size: 0.000002 mm
 

Reproduction:
Viruses cannot reproduce without the assistance of a host cell. They infect the host cell and then use their genes to direct the cell to make more virus particles. These eventually burst from the cell, destroying it.
 

Diseases include:
Severe Acute Respiratory Syndrome (SARS) - SARS virus
HIV/AIDS- Human Immunodeficiency virus (HIV).
Influenza ('flu) - Influenza virus
German measles - Rubella virus
Cold sores - Herpes simplex
Chicken pox - Varicella zoster
Common cold - Rhinovirus
 
Protozoa

Description: 
Single-celled organisms with the ability to move independently. Some hunt and engulf microscopic food particles such as bacteria.
Protozoa contain a well-structured cytoplasm, cell membrane and clearly visible nucleus. They may also contain digestive food vacuoles.
 

Size: Up to 1mm
 

Reproduction:
Most reproduce asexually by copying cell structures and then splitting into two new cells. This binary fission common but protozoa are able to reproduce sexually with others of their species when present.
 

Diseases include:
Amoebic dysentery - Entameba Hystolytica.
Malaria - Plasmodium vivax.
Sleeping sickness - Trypanosoma brucei.
Toxoplasmosis - Toxoplasma gondii.
 
Picture 2. Bacteria.
Picture courtesy of CDC, USA.
Picture 2. Fungi.
Picture courtesy of CDC, USA.
Picture 2. Viruses.
Picture courtesy of CDC, USA.
Picture 2. Protozoan.
Picture courtesy of CDC, USA.