Microorganisms are harmfule in many ways. Some microorganisms cause disese in human beings, animals and plants. For example, in humans, bacteria cause diseases such as tuberculosis, cholera, typhoid, etc. Similarly, in plants, bacteria cause a disease called citrus canker. Such disease-causing microorganisms are called pathogens. Certain microbes that grow on our food sometimes produce toxic substances. These make the food poisonous causing serious illness and even death. For example, bread when left unused under moist conditions gets spoilt by fungus, producing a white cotton-like growth on the bread.