Key Stage 3 (KS3) Biology 2025 – 400 Free Practice Questions to Pass the Exam

Infectious diseases are primarily caused by microbes, which encompass a variety of organisms capable of causing disease. Microbes include bacteria, viruses, fungi, and parasites, all of which can invade the body and cause detrimental health effects. Each type of microbe can behave differently and lead to a range of infections, from mild to severe, depending on various factors like the pathogen's virulence and the host's immune response.

The other answers do not quite capture the primary cause of infectious diseases. Pesticides are substances used to kill pests and do not cause infectious diseases themselves. While viruses are indeed a type of microbe and can lead to infectious diseases, they represent just one category among many microbes. Genetic mutations are changes within DNA that can lead to various biological processes but are not direct causes of infectious diseases. Thus, selecting microbes encompasses all the infectious agents, making it the most accurate choice.