Germ theory of disease states that specific microscopic organisms called pathogens are responsible for causing infectious diseases in humans, animals, and plants. Before this concept was accepted, many people believed that diseases were caused by supernatural forces, imbalances in bodily fluids, or “bad air.” The development of microscopes and scientific experimentation in the seventeenth to nineteenth centuries helped scientists observe microorganisms and link them directly to illness. Pioneers such as Louis Pasteur demonstrated that microbes are present in the environment and can contaminate food and living tissues, while Robert Koch provided experimental proof by establishing relationships between particular microbes and specific diseases. According to germ theory, pathogens such as bacteria, viruses, fungi, and protozoa can invade a host organism, multiply, and disrupt normal physiological functions. Transmission can occur through direct contact, airborne droplets, contaminated food or water, vectors such as insects, or contact with infected surfaces. Once inside the host, microbes may produce toxins, damage cells, compete for nutrients, or trigger harmful