Disease – A named illness or sickness, with specific symptoms. Disease may result from poor diet or from infection (see infectious disease below)

Symptoms – The outward signs of a disease; for example a fever, or coughing or a feeling of muscle weakness

Micro-organism (also called microbe) – Living organisms so small they cannot be seen, including bacteria, viruses, fungi, algae and protozoa. Bacteria can be as large as 10 micro-metres (0.000 001m) and viruses as small as 50 nano-metres (0.000 000 05 m). Like all other organisms they feed, grow and reproduce.

Infectious (or Contagious) disease – A disease caused by a microbe (popularly called a germ), which can be passed from one host to another through fine water droplets in the air, by contact or through food and drinking water. Contagious implies transmission by direct contact.

Virus – A micro-organism which is much smaller than a single cell. Responsible for many diseases, including flu and the common cold.

Host (carrier) – A human (or plant or animal) in which a microbe is active.

Incubation period – The time during which the population of disease-causing microbes is increasing inside its host and before the host experiences symptoms of the disease. The host can infect others during this time.

Immune – Protected against disease by being able to fight the invading microbe biologically.

Vaccination – A method of artificially inducing immunity, by injecting (or ingesting) microbial material (e.g. weakened strains: dead micro-organisms and extracts of micro-organisms) that triggers production of antibodies, without making the individual seriously ill.

Epidemic – When the number of people (or animals) hosting the microbe and experiencing disease symptoms rises, making everyone in a population at risk of infection

Quarantine – Keeping a host isolated so that the microbe is not able to invade others

Public health regulations – Because the ecology of human diseases involves hosts passing dangerous microbes to others, there is legislation to provide controls and services which aim to protect human populations by minimizing the risks. For example, certain diseases are notifiable (doctors must notify their local health authority of all known cases). Public health includes hygiene regulations and, in urban areas, sewers and sewerage treatment.