Final answer:
The term for a bacterium, virus, or another microbe that causes the disease is a 'pathogen'. Pathogens include bacteria and viruses that can invade hosts and cause illnesses like tuberculosis or the common cold. The germ theory of disease explains that these microbes are responsible for causing diseases.
Step-by-step explanation:
Pathogens are microscopic entities, such as bacteria and viruses, which invade human hosts, reproduce, and can make the host organism sick, leading to disease. Such diseases are often contagious, meaning they can spread from one individual to another. Although not all bacteria cause disease, pathogenic bacteria can lead to illnesses such as tuberculosis and cholera, whereas viruses can cause diseases like influenza and the common cold. The idea that microbes cause disease is known as the germ theory of disease. This theory became widely accepted in the scientific community as research and experiments showed that practices like sanitation, and the avoidance of individuals with contagious diseases, helped to prevent the spread of illnesses. This understanding led to the development of treatments, such as antibiotics for bacterial infections and vaccines for certain viral diseases. Not all microorganisms are harmful; in fact, many bacteria are harmless or even beneficial. But when we refer to those that cause disease, we use the term pathogen to describe a bacterium, virus, or other microbes responsible for causing an infection and potentially leading to illness.