AGUE
A
recall from memory of these words are
spelled, formulation and execution of
the required hand movements, and vis-
ual checking that written words match
their representation in the brain. These
processes may take place in a number of
connected regions of the brain. Agraph-
ia may be caused by damage to any of
these regions (most commonly as a
result of a
head injury,
a
stroke,
or a
brain
tumour)
and can therefore be of differ-
ent types and degrees of severity.
Agraphia is often accompanied by
alexia
(loss of reading ability) or may be
part of an expressive
aphasia
(a general
disturbance in expression of language).
OUTLOOK
There is no specific treatment for agra-
phia, but some of the lost writing skills
may return in time.
ague
An outdated term for the fever in
mala-
ria
and similar diseases, in which the
sufferer alternately feels excessively hot
and shiveringly cold.
AIDS
The abbreviation for acquired immune
deficiency syndrome, a deficiency of
the
immune system
due to infection with
the
human
immunodeficiency
virus
(see
HIV).
The interval between infec-
tion and the development of AIDS is
highly
variable.
Without
treatment,
around half of those individuals in-
fected will develop AIDS within eight to
nine years. In about one in ten cases,
however, progression to AIDS is very
slow, taking up to 20 years or longer.
Illness and death from AIDS is a grow-
ing health problem worldwide, and
there is, as yet, no cure or vaccine.
METHODS OF TRANSMISSION
HIV is transmitted in body fluids such
as semen, blood, vaginal secretions, and
breast milk. Major methods of transmis-
sion are sexual contact (vaginal, anal, or
oral), blood to blood (via transfusions,
or needle-sharing in drug users), and
mother-to-fetus.
HIV
has
also
been
transmitted
through
blood
products
given to treat
haemophilia,
kidney trans-
plants, and artificial insemination by
donated semen; but improved screening
has greatly reduced these risks. HIV is
not spread by everyday contact, such as
hugging or sharing crockery.
EFFECTS OF THE VIRUS
The virus enters the bloodstream and
infects cells with a particular receptor,
called the CD4 receptor, on their sur-
face. These cells include a type of white
blood cell called a CD4 lymphocyte (a
T lymphocyte with a CD4 receptor), that
is responsible for fighting infection, and
cells in other tissues such as the brain.
The virus reproduces within the infected
cells, which then die, releasing more
virus particles into the blood. If the in-
fection is left untreated, the number of
CD4
lymphocytes
falls,
resulting
in
greater susceptibility to certain infec-
tions and some types of cancer.
SYMPTOMS AND SIGNS
Some people experience a short-lived
illness similar to infectious
mononuc-
leosis
when they are first infected with
HIV. Many individuals have no obvious
symptoms. After the initial illness, many
people remain well. Some may suffer
from enlarged lymph nodes, muscle
pain,
and excessive sweating.
Severe
bacterial infections, such as pneumonia,
are common. Later, vague complaints,
such as weight loss, fevers, sweats, or
unexplained
diarrhoea
(described as
AIDS-related complex) may herald the
development of AIDS.
Other features of infection with HIV
include skin disorders and a variety of
viral, fungal and bacterial infections.
HIV may also affect the brain, causing
neurological disorders such as
dementia.
Certain conditions, known as AIDS-
defining illnesses, mark the develop-
ment of full-blown AIDS. These include
cancers (lymphoma of the brain,
Kap-
osi’s sarcoma,
and cancer of the
cervix)
and
various
infections
(pneumocystis
pneumonia,
cytomegalovirus
infection,
toxoplasmosis,
diarrhoea as a result of
C
ryptosporidium
or I
sospora
, candidiasis,
disseminated
strongyloidiasis,
and
crypto-
coccosis),
many of which are described
as
opportunistic infections.
DIAGNOSIS
Confirmation of HIV infection involves
testing a blood sample for the presence
of antibodies to HIV
(see
HIV test),
which
may
not
develop
for
three
months after initial infection. The con-
dition is monitored using blood tests
that measure the number of CD4 lym-
phocytes in the blood or by measuring
viral
load
(the
amount
of
virus
detectable in the blood). Diagnosis of
full-blown AIDS is based on a positive
HIV test along with the presence of an
AIDS-defining illness.
TREATMENT AND OUTLOOK
Treatment of HIV infection with a com-
bination of
antiviral drugs
can slow the
progress of the disease, and may prevent
the development of full-blown AIDS.
The main types of antiviral drug used
are
protease inhibitors,
such as indinavir,
and
reverse transcriptase inhibitors
such as
zidovudine.
Several
drugs are usually
used together to prevent resistance from
developing. AIDS-defining illnesses are
treated as they develop.
Since the introduction of antiviral
drug
combination
therapies,
deaths
from AIDS in the developed world have
been reduced dramatically. HIV infec-
tion remains life-threatening, however,
and the
most
effective
strategy
for
defeating it is prevention of infection.
PREVENTION OF INFECTION
The risk of infection can be reduced by
practising
safer sex.
and by intravenous
drug users not sharing needles. There is
a small risk to health workers handling
infected needles or blood products, but
this can be minimized by the adoption
of safe practices in the workplace.
AIDS-related complex
A combination of symptoms including
weight loss, fever, neurological prob-
lems, and recurrent infections in an
individual who has been infected with
HIV
(the virus that causes
AIDS)
but has
not yet developed AIDS. Many people
with AIDS-related complex will eventu-
ally develop the features of AIDS.
air
The colourless, odourless mixture of
gases that forms the Earth’s atmosphere.
Air consists of 78 per cent
nitrogen,
21
per cent
oxygen,
small quantities of
car-
bon dioxide
and other gases, and some
water vapour.
The balance of atmospheric gases is
maintained largely by the mutual needs
of plants and animals. Plants use carbon
dioxide and release oxygen in a process
called photosynthesis; animals use oxy-
gen during respiration, and produce
carbon dioxide as a waste product;
However, the level of carbon dioxide in
the atmosphere is gradually increasing
as a result of large-scale deforestation
and the burning of fossil fuels, which
may lead to significant global warming,
also known as the greenhouse effect.
(See also
pollution.)
air conditioning
A system that controls the temperature,
humidity, and purity, of the air in a
building. Contaminated air-condition-
ing
systems
may
cause
legionnaires’
disease
(a type of pneumonia)
and
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