Universally, approximately 20% of the cancers are related to infectious agents. Intracellularly, viruses encode proteins to reprogramme signalling pathways for cell proliferation, differentiation, dissolution, genomic integrity and immune assimilation. Viral genomes are tightly crammed within the host structure to target cellular regulatory modes. Mostly these are key proteins and are subject to mutation and chromosomal re-arrangements in non-viral diseases and carcinomas. Many retroviruses integrate near c-oncogenes and express by proviral insertional mutagenesis and modulate cell growth and differentiation. Retroviruses that carry the viral oncogenes can engender a wide variety of neoplasia (abnormal growth and proliferation of abnormal cells or abnormal amount of cells which exceeds and is uncoordinated with that of the normal tissue and persists in the same excessive manner after the cessation of the stimuli which evoked the conversion, due to a benign or malignant process) in a short interval, chiefly haematopoietic and mesenchymal malignancies etc.