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

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

M13 bacteriophage emerging from infected E.coli without lysis.

Illustration of hepatitis C virus (Flavivirus), an RNA virus.

Molecular model of the HIV virus.

Cutaway view of HIV virus surrounded by whole HIV viruses.

Cutaway view of HIV virus.

Electron micrograph of parainfluenza virus, part of the paramyxovirus group. It is generally larger than the myxovirus. It effects the respiratory system, mag. 51,300x.

T4 bacteriophage (bacterial virus) attacking E-coli bacteria.

Illustration of a multipolar neuron, a type of neuron that possesses a single axon and many dendrites. This allows for the integration of a great deal of information from other neurons. Multipolar neurons constitute the majority of neurons in the brain and include motor neurons and interneurons.

Illustration of a multipolar neuron, a type of neuron that possesses a single axon and many dendrites. This allows for the integration of a great deal of information from other neurons. Multipolar neurons constitute the majority of neurons in the brain and include motor neurons and interneurons.

Diagram showing structure and size of a typical Rabies virus (Rhabdovirus), causing Vesicular stomatitis and certain plant virus diseases.

Illustration of the concept of biological organization.

An ' stylized depiction of a retrovirus, consisting of a geomic nucleic acid, RNA or DNA (here RNA, in violet), and surrounded by capsid proteins (orange-yellow), surrounded by a glycoprotein envelope. Examples of retroviruses are the flu virus, the AIDS virus, and the SARS virus (depicted here). Viruses can only reproduce by invading and taking over other cells, as they lack the cellular machinery for self-reproduction.

PHOTO: EAST NEWS/BSIP This scanning electron micrograph (SEM) revealed some of the ultrastructural morphologic features of a Klebsiella pneumoniae bacterium. K. pneumoniae is a non-motile, Gram-negative rod, and a facultative anaerobe, which means that it is able to adapt to an existence in an oxygenated or deoxygenated environment. It is an encapsulated bacterium that is commonly found in the human digestive tract, and is often the cause of hospital acquired, or nosocomial infections involving the urinary and pulmonary systems.

PHOTO: EAST NEWS/BSIP This electron micrograph reveals a number of RNA rotavirus virions, and a number of unknown, 29nm virion particles. A rotavirus has a characteristic wheel-like appearance when viewed by electron microscopy, i.e., rotavirus is derived from the Latin rota, meaning "wheel". Rotaviruses are nonenveloped, double-shelled viruses, making the virus stable in the environment.

Illustration of streptococcus (Streptococcus pneumoniae) bacteria, schematically depicted in coccoidal chains and diplococci.

Illustration of a cutaway cross section of an HIV virus. Envelope proteins (exterior, purple and yellow) attach to the phospholipid bilayer (green) that surrounds the matrix proteins (blue). Capsule proteins (interior, purple) make up the capsule that contains two copies of the viral RNA (red) and two molecules of reverse transcriptase enzyme (orange).

Illustration of a cutaway cross section of an HIV virus. Envelope proteins (exterior, purple and yellow) attach to the phospholipid bilayer (green) that surrounds the matrix proteins (blue). Capsule proteins (interior, purple) make up the capsule that contains two copies of the viral RNA (red) and two molecules of reverse transcriptase enzyme (orange).

Bacteriorhodopsin and lipid molecules. Bacteriorhodopsin is a seven-helical transmembrane protein found in the cellular membrane of halobacteria, where it acts as a light-driven proton pump.