VISION, DISORDERS OF
VIRUSES AND DISEASE
All viruses have the same basic
structure (right), but they come in
various shapes and sizes. Examples
from the main families are shown
below (some in cross-section).
All are tiny - from about 15 to 300
nanometres in diameter (one
nanometre equals one thousand-
millionth of a metre); most are so
small that they can be seen only with
an electron microscope. All types of
viruses can multiply only within cells
of their host (far right).
Structure of a typical virus particle
Nucleic acid in the centre is surrounded by
one or more capsids made of protein subunits.
Examples of conditions or diseases
Warts, cervical cancers, anal cancers
Respiratory and eye infections
Cold sores, genital herpes, chickenpox, herpes zoster (shingles),
glandular fever, cytomegalovirus (CMV)
Cowpox, smallpox (eradicated), molluscum contagiosum
Poliomyelitis, viral hepatitis type A, respiratory infections, myocarditis
Rubella, yellow fever, dengue, encephalitis
Mumps, measles, respiratory syncytial virus (RSV)
AIDS, some leukaemias
The sequence below shows how
a virus multiplies. The signs and
symptoms ofviral infection are
caused by the virus interfering with
or destroying the host’s cells.
The virus particle first attaches
itselfto and then injects itself
into the host cell.
The viral capsid breaks down
and the viral nucleic acid (DNA
or RNA) contained inside is released.
The viral nucleic acid replicates itself;
the new copies are made from raw
materials in the host cell.
Each of the new copies ofthe
viral nucleic acid now directs
the manufacture of a capsid for itself.
The newly formed virus particles
are released in large numbers,
and the host cell may be destroyed.