Artist's concept of the life cycle of the paramyxovirus that causes mumps. Cross section of the virus and stages of development within a host cell are labeled.

Artist's concept of the life cycle of the paramyxovirus that causes mumps. Stages of development within a host cell are labeled.

Illustration showing a cancerous breast cell producing too many HER genes, which in turn produce an excessive number of HER2 receptors. The receptors attract growth factors, which stimulate the growth of more cells. Herceptin (generic name, trastuzumab) and other antibodies inhibit HER2 receptors and VEGF growth factors. The growth signal is green.

Close-up illustration showing a cancerous breast cell producing too many HER genes, which in turn produce an excessive number of HER2 receptors. The receptors attract growth factors, which stimulate the growth of more cells. Herceptin (generic name, trastuzumab) and other antibodies inhibit HER2 receptors and VEGF growth factors.

Illustration of an HIV virus, the virus that causes AIDS, shown cutaway.

Illustration of an HIV virion.

3-D space-filling molecular model of the dengue virus (family Flaviviridae, genus Flavivirus) protein capsid, bead chain version, showing the structure of how the individual protein subunits fit together, diameter 488 angstroms.

3-D space-filling molecular model of the dengue virus (family Flaviviridae, genus Flavivirus) protein capsid, bead chain version, showing the structure of how the individual protein subunits fit together, diameter 488 angstroms.

3-D space-filling molecular model of the dengue virus (family Flaviviridae, genus Flavivirus) protein capsid, bead chain version, showing the structure of how the individual protein subunits fit together, diameter 488 angstroms.

3-D space-filling molecular model of the dengue virus (family Flaviviridae, genus Flavivirus) protein capsid, diameter 488 angstroms.

3-D space-filling molecular model of the dengue virus (family Flaviviridae, genus Flavivirus) protein capsid, diameter 488 angstroms.

Illustration of a prokaryotic cell with labeled organelles: capsule, plasma membrane, cell wall, falgellum, ribosomes, and nucleoid.

Adenovirus, one of a large group of viruses causing disease of the upper respiratory tract and conjunctiva.

Molecular structures of adenovirus, one of a large group of viruses causing disease of the upper respiratory tract and conjunctiva.

Elements of viral and bacterial infection: Helicobacter pylori, streptococcus bacteria, influenza, HIV, and bacteriophage viruses.

Completely assembled, new retroviruses on cell's surface bud off into bloodstream, free to find and attack other host cells.

Retrovirus moves across protein-studded surface of a white blood cell, contacting a spiky receptor to which one of its own knobby proteins stick. This chemically unlocks the cell, which may do little to defend itself.

Molecular model of the adenovirus (common cold virus).

Computerized model of the common cold virus.

Illustration of adenovirus, which causes various respiratory infections in humans.