Many microbial agents can commonly move from an affected person to someone else in a variety of ways. In other words, they can be ‘communicated’, and so are also called communicable diseases.
Such disease-causing microbes can spread through the air. This occurs through the little droplets thrown out by an infected person who sneezes or coughs. Someone standing close by can breathe in these droplets, and the microbes get a chance to start a new infection. Examples of such
diseases spread through the air are the common cold, coronavirus disease (COVID-19), pneumonia and tuberculosis.
We all have had the experience of sitting near someone suffering from a cold and catching it ourselves. Obviously, the more crowded our living conditions are, the more likely it is that such airborne diseases will spread.
Diseases can also be spread through water. This occurs if the excreta from someone suffering from an infectious gut disease, such as cholera, get mixed with the drinking water used by people living nearby. The cholera causing microbes will enter a healthy person through the water they drink and cause disease in them. Such diseases are much more likely to spread in the absence of safe supplies of drinking water.
The sexual act is one of the closest physical contact two people can have with each other. Not surprisingly, there are microbial infections such as syphilis or AIDS that are transmitted by sexual contact from one partner to the other. However, such sexually transmitted diseases are not spread by casual physical contact. Casual physical contacts include handshakes or hugs or sports, like wrestling, or by any of the other ways in which we touch each other socially. Other than the sexual contact, the virus causing AIDS (HIV) can also spread through blood-to-blood contact with infected people or from an infected mother to her baby during pregnancy or through breast feeding.
We live in an environment that is full of many other creatures apart from us. It is inevitable that many diseases will be transmitted by other animals. These animals carry the infecting agents from a sick person to another potential host. These animals are thus the intermediaries and are called vectors. The commonest vectors we all know are mosquitoes. In many species of mosquitoes, the females need highly nutritious food in the form of blood in order to be able to lay mature eggs. Mosquitoes feed on many warm-blooded animals, including us. In this way, they can
transfer diseases from person to person.