Bacteriophages can be DNA or RNA viruses that only infect prokaryotic organisms.

They are formed only by nucleocapsid, there are no enveloped shapes. The most studied are those that infect the intestinal bacteria. Escherichia coli, known as phages T. These are constituted by a very complex protein capsule, which has a region called head, polygonal shape, involving a DNA molecule, and a region called tail, cylindrical shape, containing, at its free end, protein fibers.

Reproduction or replication of bacteriophages, as well as other viruses, occurs only within a host cell.

There are basically two types of reproductive cycles: the lytic cycle and the lysogenic cycle. These two cycles start with the phage T adhering to the bacterial cell surface through the tail protein fibers. It contracts, pushing the tubular central part into the cell like a microsyringe. The virus DNA is then injected out of the cell into the empty protein capsule. From that moment on, the differentiation between the lytic cycle and the lysogenic cycle begins.

At the lithic cycle, the virus invades the bacterium, where its normal functions are disrupted in the presence of virus nucleic acid (DNA or RNA). This one, while being replicated, commands the synthesis of the proteins that will make up the capsid. The capsids organize themselves and surround the nucleic acid molecules. New viruses are produced then. Lysis occurs, that is, the infected cell breaks down and new bacteriophages are released. Symptoms caused by a virus that reproduces this way in a multicellular organism appear immediately. In this cycle, viruses use the cell's biochemical equipment (Ribosome) to make their protein (Capsid).

At the lysogenic cycle, the virus invades the bacterium or host cell, where viral DNA incorporates into the infected cell's DNA. That is, viral DNA becomes part of the DNA of the infected cell. Once infected, the cell continues its normal operations such as reproduction and cell cycle. During the process of cell division, the genetic material of the cell, along with the virus genetic material that has been incorporated, is duplicated and then evenly divided among the daughter cells. Thus, once infected, a cell will begin transmitting the virus whenever it goes through mitosis and all cells will be infected as well. Symptoms caused by a virus that reproduces itself in this way in a multicellular organism may take time to appear. Diseases caused by lysogenic viruses tend to be incurable. Some examples include AIDS and herpes.

Under certain natural and artificial conditions (such as ultraviolet radiation, X-rays or certain chemical agents), a lysogenic bacterium can turn into non-lysogenic bacteria and start the lytic cycle.