Virology is the study of viruses – submicroscopic, parasitic particles of genetic material contained in a protein coat and virus-like agents. It focuses on the following aspects of viruses: their structure, classification and evolution, their ways to infect and exploit host cells for reproduction, their interaction with host organism physiology and immunity, the diseases they cause, the techniques to isolate and culture them, and their use in research and therapy. Virology is considered to be a subfield of microbiology or of medicine. Viruses are a major cause of death and disease. Too small to be seen by light microscopy, they were first visualized about 50 years ago by electron microscopy. The preparative techniques then available allowed only crude images to be obtained. More recently rapidly frozen specimens embedded in a glass-like form of ice have allowed detailed molecular structures to be determined. This new approach depends on sophisticated computer programs to analyse the micrographs and create three dimensional maps of specimens such as viruses. Virology is the complexes of nucleic acids and proteins that have the capacity for replication in animal, plant and bacterial cells. To replicate themselves, viruses usurp functions of the host cells on which they are parasites. The viral parasite causes changes in the cell, particularly its antigenicity; moreover, directing the host cell's metabolism to the production of new virus particles may cause cellular death. Virally-induced cell death, changes in antigenicity and the response of the host to the presence of the virus leads to the manifestations of viral disease. Viruses come in two basic types, those that have a genome of DNA and those that have a genome of RNA.