is due to the presence of a vast number of very tiny living cells called germs, or microbes, or bacteria. These little creatures are so small that it requires very high magnifying powers in a microscope to see them, but it is by their action on the living tissues of plants and animals that many diseases are produced. These germs are so small, and so light, that they can be carried about in the air breathed out from our lungs, so that they may contaminate the atmosphere or our food, and so spread disease wherever they go. That is what is meant by carrying infection. Thus the germs which cause typhoid fever or diphtheria often get into a milk supply or a water supply, and so cause an epidemic amongst all the people who use that source of water or milk.
There are many diseases which are not infectious, because they are not caused by these germs. For instance, many diseases are due to various forms of violence and pressure. This may be caused by lack of blood, or by blood being stopped from circulating properly. Still other diseases are due to various chemical substances which act as poisons upon the tissues of the body, while others are the results of extremes of heat and cold. But all these concern only the individual to which they apply at the time, and are not capable of being transmitted to somebody else, as are the diseases caused by bacteria.
Infectious disease (Wikipedia)