Micro-organisms are of different kinds and are found everywhere. They live in almost any kind of environment - in hot deserts, polar ice caps, salt water, marsh land and even inside other organisms. In every gram of soil there are about 100 million bacteria, while the collections of dead cells on our scalp contain about 500 million bacteria. The air, clothes, skin, fingernail scrapings and gut, among other places, shelter micro-organisms.
Certain micro-organisms are harmless to human beings. But others are harmful and cause different diseases to us. Such micro-organisms are called Pathogens (Patho = disease; genic = causing).