Virus, infectious agent found in virtually all
life forms, including humans, animals, plants, fungi, and bacteria. Viruses
consist of genetic material— DNA or RNA —surrounded by a protective coating of
protein, called a capsid, with or
without an outer lipid envelope. Viruses are between 20-100 times smaller than bacteria (too small to be seen
by light microscopy). Viruses vary in size from the largest poxviruses of about
450 nanometers in length to the smallest
polioviruses of about 30 nanometers. Viruses are not really free-living as they
cannot reproduce outside of a living cell;
they have evolved to transmit their genetic information from one cell to
another for the purpose of replication.
Viruses can damage or kill the cells that
they infect, causing disease in infected organisms. A few stimulate cells to grow
uncontrollably and produce cancers. Many
infectious diseases that are caused by viruses have no cures. The difficulty in
developing antiviral therapies stems from the large number of variant viruses
that can cause the same disease, as well as the inability of drugs to disable a
virus without disabling healthy cells.
Structure
and Classification Individual viruses,
or virus particles, also called virions,
contain genetic material in one of several form. Like cell DNA, almost all viral DNA is
double-stranded, and it can have either a circular or a linear arrangement.
Almost all viral RNA is single-stranded; it is usually linear, and it may be
either segmented (with different genes on different RNA molecules) or
nonsegmented (with all genes on a single piece of RNA).
The viral
protective shell, or capsid, can be either helical or icosahedral
(20 triangular sides). Capsids are composed of repeating units of one or a few
different proteins. These units are called capsomers.
Viruses also
carry genes for making proteins that are never incorporated into the virus
particle and are found only in infected cells. These viral proteins are called
non-structural proteins; they include factors required for the replication of
the viral genome and the production of the virus particle.
Some
icosahedral and helical animal viruses are enclosed in a lipid envelope
acquired when the virus buds through host-cell membranes. Inserted into this
envelope are glycoproteins that the
viral genome directs the cell to make; these molecules bind virus particles to
susceptible host cells.
The most elaborate viruses are the bacteriophages, which use
bacteria as their hosts. Some bacteriophages resemble an insect with an
icosahedral head attached to a tubular sheath. From the base of the sheath
extend several long tail fibers that help the virus attach to the bacterium and
inject its DNA to be replicated and to direct capsid production and virus
particle assembly inside the cell
Fungi
Fungi, diverse group of either single-celled or
multicellular organisms that obtain food by direct absorption of nutrients. The
food is dissolved by enzymes that the fungi excrete, is then absorbed through
thin cell walls, and is distributed by simple circulation, or streaming, of the
protoplasm.
Fungus Infections, diseases caused by the growth of fungi in or on the
body. In most healthy people fungal infections are mild, involving only the
skin, hair, nails, or other superficial sites, and they clear up spontaneously.
They include the familiar ringworm and athlete's foot. In someone with an
impaired immune system, however, such infections, called dermatophytoses, can
persist for long periods. The organisms causing dermatophytoses belong to the
genera Microsporum, Epidermophyton,
and Trichophyton.
