The existence of submicroscopic infectious agents was suspected by the end of the 19th cent. Later studies of virus crystals established that the crystals were composed of individual virus particles, or virions. Typically the protein coat, or capsid, of an individual virus particle, or virion, is composed of multiple copies of one or several types of protein subunits, or capsomeres.
Some viruses contain enzymes, and some have an outer membranous envelope.
Certain viruses, such as bacteriophages, have complex protein tails. The nucleic acid specifies information for the synthesis of from a few to 50 different proteins, depending on the type of virus. Viral Infection of a Host Cell A free virus particle may be thought of as a packaging device by which viral genetic material can be introduced into appropriate host cells, which the virus can recognize by means of proteins on its outermost surface. A bacterial virus infects the cell by attaching fibers of its protein tail to a specific receptor site on the bacterial cell wall and then injecting the nucleic acid into the host, leaving the empty capsid outside. Within the cell the virus nucleic acid uses the host machinery to make copies of the viral nucleic acid as well as enzymes needed by the virus and coats and enveloping proteins, the coat proteins of the virus.