Virology is considered to be a subfield of microbiology that deals with viruses and viral diseases. Infectious diseases are mainly caused by micro-organisms like bacteria, viruses, fungi or parasites and pathogens. Some of these organisms cause diseases. These infections are mainly caused by germs. we can get infected by touching, eating, drinking or breathing something. Some of the diseases like measles and chickenpox can also be prevented by vaccines. Hand washing is also used to prevent infectious diseases.