It is defined as a complex and smallest entity which depends upon another living cell for its survival and divides according to the condition of other cell without growth and division. Most common viral diseases are small pox and polio. It was Pasteur who coined the term virus. Virus cannot grow on non living media or one can say that virus cannot survive alone. It is obligate parasite and always requires living cell to spread further. It is immobile and requires a vector for transfer.