Viruses and responses to viral infection WORLD OF MICROBIOLOGY AND IMMUNOLOGY584•
smallpox struck 5,889 of the city’s 12,000 inhabitants, killing
15% of those infected.
Edward Jenner(1749–1823) is credited with developing
the first successful vaccineagainst a viral disease, and that dis-
ease was smallpox. A vaccine works by eliciting an immune
response. During this immune response, specific immune
cells, called memory cells, are produced that remain in the
body long after the foreign microbe present in a vaccine has
been destroyed. When the body again encounters the same
kind of microbe, the memory cells quickly destroy the
microbe. Vaccines contain either a live, altered version of a
virus or bacteria, or they contain only parts of a virus or bac-
teria, enough to elicit an immune response.
In 1797, Jenner developed his smallpox vaccine by tak-
ing infected material from a cowpox lesion on the hand of a
milkmaid. Cowpox was a common disease of the era, trans-
mitted through contact with an infected cow. Unlike smallpox,
however, cowpox is a much milder disease. Using the cowpox
pus, he inoculated an eight-year-old boy. Jenner continued his
vaccination efforts through his lifetime. Until 1976, children
were vaccinated with the smallpox vaccine, called vaccinia.
Reactions to the introduction of the vaccine ranged from a
mild fever to severe complications, including (although very
rarely) death. In 1976, with the eradication of smallpox com-
plete, vaccinia vaccinations for children were discontinued,
although vaccinia continues to be used as a carrier for recom-
binant DNA techniques. In these techniques, foreign DNA is
inserted in cells. Efforts to produce a vaccine for HIV, for
instance, have used vaccinia as the vehicle that carries specific
parts of HIV.
Herpesviruses are enveloped, double-stranded DNA
viruses. Of the more than 50 herpesviruses that exist, only
eight cause disease in humans. These include the human her-
pes virus types 1 and 2 that cause cold sores and genital her-
pes; human herpes virus 3, or varicella-zoster virus (VZV),
that causes chickenpox and shingles; cytomegalovirus
(CMV), a virus that in some individuals attacks the cells of the
eye and leads to blindness; human herpes virus 4, or Epstein-
Barr virus(EBV), which has been implicated in a cancer
called Burkitt’s lymphoma; and human herpes virus types 6
and 7, newly discovered viruses that infect white blood cells.
In addition, herpes B virus is a virus that infects monkeys and
can be transmitted to humans by handling infected monkeys.
Adenovirusesare viruses that attack respiratory, intes-
tinal, and eye cells in animals. More than 40 kinds of human
adenoviruses have been identified. Adenoviruses contain dou-
ble-stranded DNA within a 20-faceted capsid. Adenoviruses
that target respiratory cells cause bronchitis, pneumonia, and
tonsillitis. Gastrointestinal illnesses caused by adenoviruses
are usually characterized by diarrhea and are often accompa-
nied by respiratory symptoms. Some forms of appendicitis are
also caused by adenoviruses. Eye illnesses caused by aden-
oviruses include conjunctivitis, an infection of the eye tissues,
as well as a disease called pharyngoconjunctival fever, a dis-
ease in which the virus is transmitted in poorly chlorinated
swimming pools.
Human papoviruses include two groups: the papilloma
viruses and the polyomaviruses. Human papilloma viruses(HPV) are the smallest double-stranded DNA viruses. They
replicate within cells through both the lytic and the lysogenic
replication cycles. Because of their lysogenic capabilities,
HPV-containing cells can be produced through the replication
of those cells that HPV initially infects. In this way, HPV
infects epithelial cells, such as the cells of the skin. HPVs cause
several kinds of benign (non-cancerous) warts, including plan-
tar warts (those that form on the soles of the feet) and genital
warts. However, HPVs have also been implicated in a form of
cervical cancer that accounts for 7% of all female cancers.
HPV is believed to contain oncogenes, or genes that
encode for growth factors that initiate the uncontrolled growth
of cells. This uncontrolled proliferation of cells is called can-
cer. When the HPV oncogenes within an epithelial cell are acti-
vated, they cause the epithelial cell to proliferate. In the cervix
(the opening of the uterus), the cell proliferation manifests first
as a condition called cervical neoplasia. In this condition, the
cervical cells proliferate and begin to crowd together.
Eventually, cervical neoplasia can lead to full-blown cancer.
Polyomaviruses are somewhat mysterious viruses.
Studies of blood have revealed that 80% of children aged five
to none years have antibodies to these viruses, indicating that
they have at some point been exposed to polyomaviruses.
However, it is not clear what disease this virus causes. Some
evidence exists that a mild respiratory illness is present when
the first antibodies to the virus are evident. The only disease
that is certainly caused by polyomavirses is called progressive
multifocal leukoencephalopathy (PML), a disease in which the
virus infects specific brain cells called the oligodendrocytes.
PML is a debilitating disease that is usually fatal, and is
marked by progressive neurological degeneration. It usually
occurs in people with suppressed immune systems, such as
cancer patients and people with AIDS.
The hepadnavirusescause several diseases, including
hepatitis B. Hepatitis B is a chronic, debilitating disease of the
liver and immune system. The disease is much more serious
than hepatitis A for several reasons: it is chronic and long-last-
ing; it can cause cirrhosis and cancer of the liver; and many
people who contract the disease become carriers of the virus,
able to transmit the virus through body fluids such as blood,
semen, and vaginal secretions.
The hepatitis B virus (HBV) infects liver cells and has
one of the smallest viral genomes. A double-stranded DNA
virus, HBV is able to integrate its genome into the host cell’s
genome. When this integration occurs, the viral genome is
replicated each time the cell divides. Individuals who have
integrated HBV into their cells become carriers of the disease.
Recently, a vaccine against HBV was developed. The vaccine
is especially recommended for health care workers who
through exposure to patient’s body fluids are at high risk for
infection.
Parvoviruses are icosahedral, single-stranded DNA
viruses that infect a wide variety of mammals. Each type of
parvovirus has its own host. For instance, one type of par-
vovirus causes disease in humans; another type causes dis-
ease in cats; while still another type causes disease in dogs.
The disease caused by parvovirus in humans is called ery-
thremia infectiosum, a disease of the red blood cells that iswomi_V 5/7/03 11:00 AM Page 584