Viruses are non-cellular obligate pathogen each has a nucleic acid core surrounded by a protein coat. The nucleic acid present in viruses is either DNA or RMA and contains complete- genetic information. The protein coat of virus is called capsid and it is formed by similar units called capsomeres Some viruses have a lipid bilayer membrane around them these are called enveloped viruses while the other viruses are called naked During penetration withing a host, fusion with the host membrane is preferred by the enveloped viruses while endocytosis is preferred by necked viruses