There are various bacteria, viruses, protozoa, and fungi that act as pathogens and infect the host organism. They can be found in air, dust, surfaces or soil, etc. Several bacterial species are pathogens for various bacterial diseases and cause bacterial infection such as mycobacterium tuberculosis causes tuberculosis, streptococcus pneumonia causes pneumonia, etc. however not all bacterial species are pathogenic. Most of them are harmless and termed as good bacteria or probiotics. Pathogenic viruses also cause different diseases in humans and other animals, for example- smallpox, influenza, measles, chickenpox, and rubella.