Parasitism is a relationship between two living organisms of different species in which one organism called parasite obtains its food directly from another living organism called host. Majority of the parasites harm the host; they may reduce the survival, growth and reproduction of the host and reduce its population density. Pathogens are disease-causing microorganisms which get benefit by causing harm to host organism. Pathogens such as viruses, bacteria, fungi, protozoans, helminths, insects etc., reproduce or multiply inside the host organism and therefore, they get the opportunity to complete the life cycle and spread their population.