What is an infectious disease?

An infectious disease is defined as a illness caused by harmful organisms such as bacteria, viruses, fungi, or parasites. These organisms can invade the body and multiply, potentially leading to various health issues. Importantly, many infectious diseases are capable of being transmitted from one individual to another, although some may also arise from organisms that exist normally in the environment without necessarily spreading between people.

This option embodies the comprehensive nature of infectious diseases, highlighting both the role of harmful organisms and the possibility of transmission. For example, diseases like the flu or tuberculosis fall within this category as they can spread from person to person, while diseases like those caused by certain environmental bacteria may not.

The other options differentiate infectious diseases from conditions linked to non-infectious causes, such as genetic factors, toxins, or lifestyle choices, underscoring the unique characteristics of infectious diseases related to microbial pathogens and their potential for spread.