What is an infectious disease caused by?

An infectious disease is caused by pathogenic or harmful microorganisms. These microorganisms include bacteria, viruses, fungi, and parasites, which invade the body and can multiply, leading to illness. When these pathogens enter a host, they can disrupt normal bodily functions, causing various symptoms and health issues. The ability of these microorganisms to cause disease is attributed to their pathogenicity, which includes factors like virulence, the ability to adhere to host tissues, and mechanisms of evading the immune response.

Non-pathogenic microorganisms do not cause disease and are often part of the normal flora found in and on the body. Environmental factors may influence the occurrence or spread of infectious diseases, but they do not directly cause them. Genetic mutations can lead to various disorders or impact how the body responds to infections but are not the cause of infectious diseases themselves. Thus, the focus remains on the role of pathogenic microorganisms in the onset of infectious diseases.