Which of the following best defines an infectious disease?

An infectious disease is best defined as a disorder caused by microorganisms that spread. This definition encompasses a wide range of diseases caused by pathogens such as bacteria, viruses, fungi, and parasites. Infectious diseases can be transmitted from one individual to another, through direct contact, airborne transmission, vector-borne routes, or by consuming contaminated food or water. This ability to spread and cause illness in individuals makes infectious diseases a significant area of concern for public health.

The other definitions do not accurately describe infectious diseases. For instance, a physical injury from an external force is not related to pathogens but rather to trauma. Chronic conditions linked to genetics involve long-term health issues that are not typically caused by infections but are rather a result of hereditary factors. Lastly, allergic reactions are responses of the immune system to environmental substances and do not involve infections caused by microorganisms. Thus, the correct choice highlights the core characteristics of infectious diseases, emphasizing their biological basis and transmission dynamics.