Microorganisms are harmful for human beings in many ways. They cause diseases in humans, plants and animals, e.g. In humans, bacteria cause diseases such as cholera, typhoid, tuberculosis, etc.
In cattle, the foot and mouth disease is caused by a virus. Also several microbes cause diseases in plants, e.g. Citrus canker disease, late blight of potato.
Some microbes grow on food and secrete toxic substances which make the food poisonous.