The germ theory of disease is a scientific concept that states that many diseases are caused by microorganisms, such as bacteria, viruses, and fungi. These tiny organisms can invade the body, multiply, and disrupt normal bodily functions, leading to illness. This theory was developed in the 19th century, significantly advancing our understanding of health and disease. Key figures in the development of germ theory include Louis Pasteur and Robert Koch. Their research demonstrated the link between specific germs and specific diseases, paving the way for modern medicine, including the development of vaccines and antibiotics. This understanding has been crucial in preventing and treating infectious diseases.