What characterizes infectious diseases?

Infectious diseases are primarily characterized by their origin in pathogenic microorganisms, such as bacteria, viruses, fungi, or parasites. These microorganisms can invade the body, multiply, and subsequently lead to a range of health issues, from mild to severe. The interaction between the pathogen and the host immune response plays a crucial role in the manifestation and severity of these diseases.

Understanding this characteristic helps highlight the importance of hygiene, vaccination, and public health measures to prevent the spread of infectious diseases. The other options do not accurately define infectious diseases: they either suggest incorrect conditions regarding contagion, the necessity of surgical intervention, or link the diseases solely to nutritional status, which is not a primary determining factor in infectious disease categorization.