Final answer:
An infection is the colonization of a host by microorganisms, while disease refers to any deviation from the normal function or structure of the host. Some infectious diseases are communicable or contagious, while others are noncommunicable but can be contracted through contact with environmental reservoirs or animals.
Step-by-step explanation:
An infection is the successful colonization of a host by a microorganism. Infections can lead to disease, which causes signs and symptoms resulting in a deviation from the normal structure or functioning of the host. Microorganisms that can cause disease are known as pathogens.
In an infection, a microorganism enters a host and begins to multiply. Some infections cause disease, which is any deviation from the normal function or structure of the host. Signs of a disease are objective and are measured. Symptoms of a disease are subjective and are reported by the patient.
Diseases can either be noninfectious (due to genetics and environment) or infectious (due to pathogens). Some infectious diseases are communicable (transmissible between individuals) or contagious (easily transmissible between individuals); others are noncommunicable, but may be contracted via contact with environmental reservoirs or animals (zoonoses).
Infection with parasitic microorganisms often causes signs of overt disease. Well-known examples in plants include potato blight and wheat rust, both of which are caused by a fungus. Examples in humans include malaria and dysentery, which are due to protozoa; and tuberculosis and cholera, which are due to bacteria.
The mechanisms by which disease-causing microbes cause damage to the host's tissues are variable. In some infectious diseases the injury is due to toxic substances produced by the invading organism, as in the case of diphtheria and tetanus. In others, the inflammatory response of the host's tissues is a major cause of distress, as in the case of tuberculosis, plague and pneumonia.