Many illnesses are caused by small living things, too small to see, that get into wounds and our digestive systems, which is why boiling water is a good rule.
The small life forms that sometimes make us ill are not all harmful, especially when they are in the right place, not the wrong place. Some of them are useful.
The small things that cause disease can be grouped by their appearance, their habits and in other ways as viruses, bacteria, Fungi, Protozoa and parasites.
Some sorts of illness are caused by things we eat, or by things we have failed to eat and need. We need to eat a balanced diet of different food types.
When a disease spreads widely, this is called an epidemic, but if it spreads across the world (as HIV has done), it is usually called a pandemic.
A disease spread by an animal is called a zoonosis: examples include tuberculosis, formerly spread by cattle, avian (bird) forms of influenza, and maybe SARS.
We get diseases from the bites of invertebrates like flies, mosquitoes and ticks, because the biter carries and spreads a virus, bacterium or protozoan.
To identify the cause of a disease, we study a large number of people, half with, and half without the disease, and see what else distinguishes the two groups.
Scientists use a range of standard methods to identify the cause of a disease, using maps, statistics, and a knowledge of how diseases spread in populations.
Epidemiology involves finding causes of disease and also vectors. In modern times it has come to rely heavily on genomic methods to identify causes of disease.
Most of our understanding of disease relies on the germ theory, which says that diseases are commonly caused by microbes of different sorts invading us.
The microbes that cause disease can be classed as protists (protozoans), viruses, bacteria and some Fungi. No Archaea are known to cause disease.
Some diseases are dietary deficiency diseases, as a result of the diet lacking some essential vitamin or mineral, or because the needed item cannot be retained.
Some diseases are not inherited as such, but it is possible to have a genetic predisposition, to inherit an increased vulnerability to a given disease.
The term 'drug' has two distinct meanings, one relating to drugs of addiction, the other relating to drugs used to treat illnesses. All drugs can do harm.
Aging and degenerative diseases include Alzheimer disease, Parkinsonism, arthritis, macular degeneration, and cancers caused by self-repair failures.
Disease can be caused by dust such as silica or asbestos in the lung, chemicals in the environment (including food) and pollution in various forms.
Many conditions once called environmental diseases are actually set off by microbes, including ulcers and heart disease, both formerly blamed on stress.