Communicable diseases

• • Viruses are tiny, non-living (do not contain cells) particles that can reproduce rapidly within the body.
  • They invade host cells and use them to make new virus particles.
  • Once the new viruses are made, they cause the cells to burst.
  • The cell damage makes us feel ill.
• • Bacteria are small living cells that either damage cells directly or by producing toxins (poisons).
  • Bacteria can reproduce rapidly in the appropriate conditions: warm, moist areas with a good supply of oxygen.
  • The toxins released by bacteria can damage cells and tissues, making us feel ill.
• • Some fungi are single-celled organisms that have a body made up of hyphae (thread-like structures).
  • The hyphae are able to grow and penetrate tissues, such as human skin or the surface of plants.
  • The hyphae produce spores that can spread to other plants and animals.
• • Protists are single-celled eukaryotic organisms (they have a nucleus).
  • Many are parasites, meaning that they live on or inside other organisms.
  • They are often transferred to the host organism by a vector, such as an insect or mosquito.
  • They make us feel ill by damaging our tissues.
• • Some pathogens are released into the air inside droplets when an organism coughs or sneezes. These droplets can then be inhaled (breathed in) by other organisms.
  • The flu and common colds are spread in this way.
• • Some pathogens contaminate water supplies. Organisms drinking this contaminated water can be infected by pathogens.
  • Cholera is a disease spread in this way.
• • Some pathogens live on the surface of other organisms, such as the skin. If infected skin touches a surface, some of the pathogens can transfer from the infected skin to the surface.
  • The pathogen then spreads to other people who also touch that surface. Athlete's foot is spread in this way.