Page 8 - The Science of Infectious Diseases

fi shelsoni bacterium, which measures about 80 µm in diameter
and 600 µm in length, is visible to the unaided eye.
One of the most dangerous features of infectious bacteria is
that they can live outside the body on the surfaces of objects. Not
only can bacteria live on smooth surfaces such as countertops
and doorknobs, they also can survive on sheets and towels. Be-
cause they are mobile, they can move along tubes and pipes. For
this reason, Staphylococcus aureus is a major source of infection
in hospitals, since the bacteria can enter the body by moving
down feeding tubes, breathing tubes, dialysis tubing, and urinary
catheters. According to the WHO, 49 percent of patients who
develop system-wide infections, or sepsis, in intensive care units
acquired the infection in the hospital, and 42 percent of those
patients die from the infection.

Viruses
Viruses are ten to one hundred times smaller than bacteria. Sci-
entists measure them in nanometers. A nanometer is one-billionth
of a meter. Viruses range in size from 20 to 400 nanometers in
diameter. They are too small to see with a conventional light mi-
croscope, so they are called submicroscopic. They can only be
viewed using an electron microscope.
Unlike bacteria, viruses are not cells. They do not have a cell
wall that surrounds a cytoplasm containing the organism’s genetic
material, known as deoxyribonucleic acid (DNA) and ribonucleic
acid (RNA). Instead, viruses have a core of genetic material sur-
rounded by a protein coat, known as a capsid. While a bacterium
can reproduce itself—by making a copy of its DNA and dividing
in a process called binary fi ssion or by growing a copy of itself
in a process known as budding—viruses lack the biological ma-
chinery to reproduce on their own. They are inanimate particles
of genetic material. They must enter a living host cell and use its
machinery to make copies of themselves. For this reason, viruses
are not considered to be living creatures but instead occupy a
state between living and nonliving things.

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