10
HIV/AIDS
a renewed international will to remove the
burden of malaria from the poorest parts of the
world, allied to generous donations from wealthy
individuals and foundations.
Infection with the human immunodeficiency virus (HIV)
may lead to acquired immunodeficiency syndrome
(AIDS) (Figure 10.9). The features of AIDS and HIV
are listed in Table 10.4. The features of viruses are
described in Chapter 1 (Section 1.9, Viruses).
Questions
9
Figure 10.8 shows the number of cases of malaria
and number of deaths from malaria recorded
in hospitals in Zanzibar, United Republic of
Tanzania, between 1999 and 2008. (Adapted from:
WHO World Malaria Report 2009)
a i Looking at the graph, describe the data
collected from hospitals in Zanzibar.
ii Suggest possible reasons for the pattern
you describe.
iii Suggest other data that could have
been collected in Zanzibar to assess the
effectiveness of methods to control malaria.
b Suggest why WHO recommends that cases
of suspected malaria are confirmed using the
parasite diagnostic tests before treatment is given.
Number of cases of malaria per year
12 000
10 000
Key
number of cases
number of deaths
400
300
250
6000
200
150
4000
100
2000
50
0
1999 2000 2001 2002 2003 2004 2005 2006 2007 2008
Year
protein matrix
envelope
lipid bilayer
capsid
protease
RNA
450
350
8000
HIV is a retrovirus, which means that its genetic material
is RNA, not DNA. Once inside a host cell, the viral
RNA is converted ‘back’ to DNA (hence ‘retro’) to be
incorporated into human chromosomes. The virus infects
gp 41
Number of deaths from malaria per year
•
Infectious disease
0
gp 120
reverse
transcriptase
Figure 10.9: Human immunodeficiency virus (HIV).
The outer lipid membrane forms the envelope, which
also has two glycoproteins: gp120 and gp41. The
protein core contains genetic material (RNA) and two
enzymes: a protease and reverse transcriptase. Reverse
transcriptase uses the RNA as a template to produce
DNA once the virus is inside a host cell.
Figure 10.8: Malaria in Zanzibar hospitals between 1999 and 2008.
KEY WORDS
10 List the factors that make malaria a difficult disease
to control.
11 Describe the precautions that people can take to
avoid developing malaria.
HIV: human immunodeficiency virus
AIDS: acquired immunodeficiency syndrome
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CAMBRIDGE INTERNATIONAL AS & A LEVEL BIOLOGY: COURSEBOOK
Pathogen
human immunodeficiency virus (HIV)
Methods of transmission
in semen and vaginal fluids during sexual intercourse
infected blood or blood products
contaminated hypodermic syringes
mother to fetus across placenta and at birth
mother to infant in breast milk
Global distribution
worldwide, especially in sub-Saharan Africa and South East Asia
Incubation period
initial incubation a few weeks, but up to ten years or more before
symptoms of AIDS may develop
Site of action of pathogen
T-helper lymphocytes, macrophages, brain cells
Clinical features
HIV infection – flu-like symptoms and then symptomless
AIDS – opportunistic infections including pneumonia, TB and
cancers; weight loss, diarrhoea, fever, sweating, dementia
Method of diagnosis
testing blood, saliva or urine for the presence of antibodies
produced against HIV
Estimated total number of people living
with HIV worldwide in 2017
36.9 million (approximately 67% of these in sub-Saharan Africa)
(UNAIDS estimate)
Estimated number of new cases of HIV
infection worldwide in 2017
1.8 million (UNAIDS estimate)
Estimated number of deaths from AIDSrelated diseases worldwide in 2017
940 000 (UNAIDS estimate)
Table 10.4: The features of HIV/AIDS.
and destroys cells of the body’s immune system so that
their numbers gradually decrease (Figure 10.10). These
cells, known as T-helper lymphocytes, control the immune
system’s response to infection (Chapter 11, Section 11.2,
Cells of the immune system ). When the numbers of these
cells are low, the body is unable to defend itself against
infection, so allowing a range of pathogens to cause a
variety of opportunistic infections. AIDS is not a disease;
it is a collection of these opportunistic diseases associated
with immunodeficiency caused by HIV infection.
Since HIV is an infective agent, AIDS is called an
acquired immunodeficiency to distinguish it from other
types – for example, an inherited form.
KEY WORD
opportunistic infection: an infection caused by
pathogens that take advantage of a host with
a weakened immune system, as may happen in
someone with an HIV infection
Transmission of HIV
After initial uncertainties in the early 1980s surrounding
the emergence of an apparently new disease, it soon
became clear that an epidemic and then a pandemic was
underway. The WHO estimated that, by 2017, 70 million
people had been infected with HIV and 35 million
people had died of HIV/AIDS.
HIV is a virus that is spread by intimate human
contact; there is no vector (unlike in malaria) and the
virus is unable to survive outside the human body
(unlike cholera or malarial pathogens). Transmission
is only possible by direct exchange of body fluids. In
practice, this means that HIV is spread most easily
through sexual intercourse, blood donation and the
sharing of needles by intravenous drug users. HIV
is also transmitted from mother to child across the
placenta and, more often, through the mixing of blood
during birth.
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10
a
c
b
d
Figure 10.10: A series of transmission electron micrographs
(TEMs) showing HIV budding from the surface of an
infected T-lymphocyte and becoming surrounded by an
envelope derived from the cell surface membrane of the
host cell (×176 000 ). a The viral particle first appears as a
bump, b which then buds out and c is eventually cut off.
d The outer region of dense material and the less dense
core are visible in the released virus.
The initial epidemic in North America and Europe
was among male homosexuals who practised anal
intercourse and had many sex partners, two forms of
behaviour that put them at risk. The mucous lining of
the rectum is not as thick as that of the vagina, and
there is less natural lubrication. As a result, the rectal
lining is easily damaged during intercourse and the
virus can pass from semen to blood. Having multiple
partners, both homosexual and heterosexual, allows the
virus to spread more widely.
Also at high risk of infection were haemophiliacs who
were treated with a clotting substance (factor VIII)
isolated from blood pooled from many donors. Such
blood products are now largely synthetic (Chapter 19,
Section 19.6, Genetic technology and medicine). Much
of the transmission of HIV has been by heterosexual
intercourse. This is particularly rapid in some African
states, where equal numbers of males and females are
now HIV-positive (HIV+).
HIV is a slow virus and, after infection, there may not
be any symptoms until years later. Some people who
Infectious disease
have the virus even appear not to develop any initial
symptoms, although there are often flu-like symptoms
for several weeks after becoming infected. At this stage,
a person is HIV+ but does not have AIDS.
The infections that can opportunistically develop to
create AIDS tend to be characteristic of the condition.
Two of these are caused by fungi: oral thrush caused by
Candida albicans, and a rare form of pneumonia caused
by Pneumocystis jiroveci. During the early years of the
AIDS epidemic, people in developed countries died
within 12 hours of contracting this unusual pneumonia.
Now this condition is managed much better and drugs
are prescribed to prevent the disease developing.
As the immune system collapses further, it becomes less
effective in finding and destroying cancers. A rare form
of skin cancer, Kaposi’s sarcoma, caused by a herpeslike virus, is associated with AIDS. Kaposi’s sarcoma
and cancers of internal organs are now the most likely
causes of death of people with AIDS in developed
countries, along with degenerative diseases of the brain,
such as dementias.
At about the same time that AIDS was first reported
on the west coast of the USA and in Europe, doctors
in Central Africa reported seeing people with similar
opportunistic infections. HIV/AIDS is now widespread
throughout sub-Saharan Africa from Uganda to South
Africa. It is a serious public health problem there
because HIV infection makes people more vulnerable to
existing diseases such as malnutrition, TB and malaria.
AIDS is having an adverse effect on the economic
development of countries in the region, as it affects
sexually active people in their twenties and thirties who
are also potentially the most economically productive,
and the purchase of expensive drugs drains government
funds. The World Bank estimated that AIDS had
reversed 10–15 years of economic growth for some
African states by the end of the 20th century.
Treating HIV/AIDS
There is as yet no cure for AIDS and no vaccine for
HIV. However, there has been much success in recent
years in treating people with drugs so that they can live
with HIV. Drug therapy can slow down the onset of
AIDS quite dramatically so that many HIV+ people
may now have a normal life expectancy.
However, the drugs are expensive and have a variety
of side-effects ranging from the mild and temporary
(rashes, headaches, diarrhoea) to the severe and
permanent (nerve damage, abnormal fat distribution). If
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used in combination, two or more drugs which prevent
the replication of the virus inside host cells can prolong
life, but they are not a cure. The drugs are similar to
DNA nucleotides (e.g., zidovudine is similar to the
nucleotide that contains the base thymine). Zidovudine
binds to the viral enzyme reverse transcriptase and
blocks its action. This stops the replication of the viral
genetic material and leads to an increase in some of the
body’s lymphocytes. A course of combination therapy
(taking several drugs) can be very complicated to follow.
The pattern and timing of medication through the day
must be strictly followed. People who are unable to keep
to this pattern can become susceptible to strains of HIV
that have developed resistance to the drugs.
Global scale-up of antiretroviral therapy has been the
primary contributor to a 48% decline in deaths from
AIDS-related causes, from a peak of 1.9 million in 2005
to 1.0 million in 2016. The number of children (aged
0–14 years) dying of AIDS-related illnesses has been nearly
cut in half in just six years, from 210 000 in 2010 to 120 000
in 2016. Much of the decline is due to steep reductions in
new HIV infections among children, with increased access
to treatment with antiretroviral drugs for pregnant women.
infection across the world decreased by 25% between
2001 and 2009, but only by 18% between 2010 and 2017.
Contact tracing is an important part of controlling the
spread of HIV. If a person who is diagnosed as HIV+ is
willing and able to identify the people he or she has put at
risk of infection by sexual intercourse or needle sharing,
then these people will be offered an HIV test. This test
identifies the presence of antibodies to HIV, although these
only appear several weeks after the initial infection. Home
testing kits for HIV are now available (Figure 10.11).
Injecting drug users are advised to give up their habit,
stop sharing needles or take their drug in some other
way. Needle-exchange schemes operate in some places
to exchange used needles for sterile ones to reduce the
chances of infection with HIV and other blood-borne
diseases.
Preventing HIV/AIDS
The spread of HIV/AIDS is difficult to control. The
virus’s long latent stage means it can be transmitted
by people who are HIV+ but who show no symptoms
of AIDS and do not know they are infected. The virus
changes its surface proteins, which makes it hard for
the body’s immune system to recognise it (Chapter 11,
Section 11.1, Defence against disease). This also makes
the development of a vaccine very difficult.
There has been much success in treating pregnant
women so that HIV does not infect their fetuses. This
has led to a considerable decrease in the number of
children born with HIV.
Public health measures are used to prevent the spread
of HIV. People can be educated about the spread of
the infection and encouraged to change their behaviour
so as to protect themselves and others. Condoms,
femidoms and dental dams are the only effective
methods of reducing the risk of infection during
intercourse, as they form a barrier between body fluids,
reducing the chances of transmission of the virus. Some
countries have promoted the use of condoms as well
as other measures. As a result, infection rates in these
countries have slowed and the number of new cases
reported each year has either decreased or remained the
same year on year. It is estimated that the rate of HIV
Figure 10.11: A home testing kit for HIV.
Blood collected from blood donors is screened for HIV and
heat-treated to kill any viruses. People who think they may
have been exposed to the virus are strongly discouraged
from donating blood. In some low-income countries,
not all donated blood is tested. Anyone concerned
about becoming infected by blood transfusion during an
operation may donate their own blood before the operation
to be used instead of blood from a blood bank.
Widespread testing of a population to find people
who are HIV+ is not expensive, but governments are
reluctant to introduce such testing because of the
infringement of personal freedom. In the developed
world, HIV testing is promoted most strongly to
people in high-risk groups, such as male homosexuals,
prostitutes, injecting drug users and their sexual
partners. If tested positive, they can be given the medical
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10
and psychological support they need. In Africa and
South East Asia, the epidemic is not restricted to such
easily identifiable groups and widespread testing is not
feasible due to the difficulty of reaching the majority of
the population and organising testing. People in these
regions find out that they are HIV+ when they develop
the symptoms of AIDS.
Both viral particles and infected lymphocytes are found
in breast milk. Mother-to-child transmission is reduced
by treating HIV+ women and their babies with drugs.
However, HIV+ women in high-income countries are
advised not to breastfeed their babies, because of the
risk of transmission even if they have a secure supply
of drugs during this period. In contrast, HIV+ women
in low- and middle-income countries are advised to
breastfeed, especially if they have a secure supply of
drugs during this period, as the protection this gives the
babies against other diseases and the lack of clean water
to make up formula milk may outweigh the risks of
transmitting HIV.
Infectious disease
Questions
12 Table 10.5 shows the number of people across the
world who are estimated to be infected with HIV,
and the number of those estimated to be receiving
treatment for HIV between 2000 and 2017.
a i Calculate the percentage change in the
number of people living with HIV between
2000 and 2017.
ii Looking at Table 10.5, explain what is
meant by the phrase ‘living with HIV’.
b i Summarise the data in Table 10.5.
ii Suggest why the numbers given in the table
are estimates.
13 Suggest the types of advice which might be offered
as part of an HIV/AIDS education programme.
14 Children in Africa with sickle cell anaemia or
malaria often receive blood transfusions. Explain
how this puts them at risk of HIV infection.
15 Explain why the early knowledge of HIV infection
is important in transmission control.
Year
Numbers estimated to be living
with HIV in millions
Numbers estimated to be
receiving treatment in millions
Proportion of people living with
HIV who are receiving treatment
2000
28.9
0.8
0.03
2005
31.8
2.2
0.07
2010
33.3
7.5
0.23
2013
35.3
13.0
0.37
2014
35.9
15.0
0.42
2015
36.7
17.0
0.46
2016
36.7
19.5
0.53
2017
36.9
21.7
0.59
Table 10.5: Estimates of the numbers of people living with HIV and those receiving treatment 2000–2017.
Tuberculosis (TB)
Table 10.6 gives the main features of this disease. TB
is caused by either of two bacteria, Mycobacterium
tuberculosis (Figure 10.12) and Mycobacterium bovis.
These are pathogens that live inside human cells,
particularly in the lungs. This is the first site of infection,
but the bacteria can spread throughout the whole body
and even infect bone tissue.
Some people become infected and develop TB quite
quickly, while in others the bacteria remain inactive for
many years. It is estimated that about 30% of the world’s
population is infected with TB without showing any
symptoms of the infection; people with this inactive,
or latent, infection do not spread the disease to others.
However, the bacteria can later become active, and this is
most likely to happen when people are weakened by other
diseases, suffer from malnutrition, smoke, have diabetes,
consume large quantities of alcohol or become infected
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