Viruses are the most prevalent of all infectious agents and are also the smallest, ranging in size from 20 to 300 nm in diameter. The term “filterable agent” was used to refer to viruses before they could be visualized reliably because they pass through 0.22-µm filters that retain other organisms. All viruses are obligate intracellular parasites that depend on the host cell’s metabolism for replication. They are classified primarily by the nucleic acid content of their core (either DNA or RNA) because they contain only a single type of nucleic acid. They are also classified on the basis of the shape of their protein coat, or capsid, which is either spherical or cylindrical.1 Classification based on symptoms has proven useful to clinicians because certain viruses preferentially affect different organs, such as the skin, the nervous system, or the respiratory tract. Skin changes are often prominent manifestations of human viral infections, and the histopathologic changes associated with them vary from nonspecific features such as those seen in viral exanthemata to pathognomonic inclusion bodies in herpesvirus and cytomegalovirus (CMV) infections.1
GENERAL PRINCIPLES OF VIRAL INFECTIONS
All viral infections are initiated by entry of the infectious particle, the virion, into the host cell by pinocytosis or phagocytosis. Viruses differ in the types of cells they infect, having tropisms for different tissues, such as human papillomaviruses (HPVs), which have a tropism for epithelial cells. Viruses reach target cells through inhalation, ingestion, or direct inoculation. After entry into the cell, enzymes remove the outer layer of the virus, the capsid, and viral nucleic acid is released into the cell. This may be followed either by active viral replication, leading to an acute viral infection, or alternatively, viral nucleic acid may be incorporated into the host nucleic acid, leading to a latent infection. Different steps are required for viral replication depending on the agent in question. In some viruses, such as enteroviruses, viral RNA acts as a messenger and is almost directly infectious because it is translated immediately by host ribosomes into viral proteins. Other RNA viruses, such as influenza viruses, have noninfectious RNA that first must be transcribed into messenger RNA by a polymerase enzyme. Retroviruses, such as the human immunodeficiency viruses, are RNA viruses that contain reverse transcriptase, an enzyme that synthesizes DNA from viral RNA. DNA viruses are generally more complex and transcribe messenger RNA from their own DNA using either host cell enzymes or intrinsic viral polymerases.
In active infections, nucleic acid replication occurs, followed by the synthesis of proteins. In general, the time required for new virus production in acute infections is measured in hours, with thousands of new virions being formed in each cell. Newly formed virions are released from the infected cell and spread to adjacent ones. During this process, the cell itself may be destroyed through lysis or may only be damaged and still survive.1 Other viruses ...