Cambridge International A Level Biology Revision Guide

Cambridge International AS Level Biology
200
Many infectious diseases, such as the common cold,
measles and influenza, only affect us for a short period of
time. Others, such as tuberculosis (TB), may last a much
longer time. Indeed, in the case of HIV/AIDS, there is as
yet no cure and treatments must be taken for the whole of
a person’s life. Some infectious diseases can only spread
from one person to another by direct contact, because
the pathogen cannot survive outside the human body.
Other pathogens can survive in water, human food, faeces
or animals (including insects), and so are transmitted
indirectly from person to person. Some people may spread
a pathogen even though they do not have the disease
themselves. People like this who lack symptoms are called
carriers, and it can be very difficult to trace them as the
source of an infection.
The way in which a pathogen passes from one host to
another is called the transmission cycle. Control methods
attempt to break transmission cycles by removing the
conditions that favour the spread of the pathogen. Control
is only possible once the cause of the disease and its method
of transmission are known and understood. Vaccination is a
major control measure for many infectious diseases; it works
by making us immune so that pathogens do not live and
reproduce within us and do not then spread to others
(pages 233–234).
Worldwide importance of
infectious diseases
Five infectious diseases of worldwide importance are
discussed in detail in this chapter: cholera, malaria, HIV/
AIDS, tuberculosis (TB) and measles. Smallpox is the only
infectious disease that has so far been eradicated, and it is
discussed in Chapter 11 (pages 235–236).
The number of people infected with cholera, malaria,
HIV/AIDS, TB and measles, particularly children and
young adults, remains very high, and these diseases pose
serious public health problems now and for the foreseeable
future. This is particularly true in parts of the world
without the efficient health services available in affluent
countries.
Table 10.1 shows the causative agents of the six chosen
diseases. To control a disease, we must first know what
causes it.
A new strain of cholera appeared in 1992 but is so
far restricted to South-East Asia. The disease is on the
increase, and it is thought many cases go unreported for
fear of disruption to travel, tourism and trade. Malaria
has been on the increase since the 1970s and constitutes
a serious risk to health in many tropical countries. AIDS
was officially recognised in 1981, but the infective agent
Disease
Causative agent
Type of
(pathogen)
organism
cholera Vibrio cholerae bacterium
malaria four species of Plasmodium protoctist
HIV/AIDS
human immunodeficiency
virus (HIV)
virus
tuberculosis Mycobacterium tuberculosis
(TB)
and M. bovis
bacterium
measles a species of Morbillivirus virus
smallpox
(eradicated)
Variola virus
virus
Table 10.1 The causative agents of six infectious diseases.
(HIV) was in human populations for many years before it
was identified. The spread of HIV infection since the early
1980s has been exponential. TB, once thought to be nearly
eradicated, has shown an increase since the 1970s and is
a considerable health risk in many countries. Thanks to
vaccination, measles is a disease that is now very rare in
developed countries, but remains a serious threat to the
health of children who live in poverty in many developing
countries.
Diseases that are always in populations are described
as endemic. TB is an example of a disease endemic in the
whole human population. Malaria is endemic in tropical
and sub-tropical regions.
The incidence of a disease is the number of people
who are diagnosed over a certain period of time, usually
a week, month or year. The prevalence of a disease is the
number of people who have that disease at any one time.
An epidemic occurs when there is a sudden increase in the
number of people with a disease. A pandemic occurs when
there is an increase in the number of cases throughout
a continent or across the world. The death rate from
different diseases is referred to as mortality.
Cholera
Transmission of cholera
The features of cholera are given in Table 10.2. Cholera is
caused by the bacterium Vibrio cholerae (Figure 10.2).
Figure 10.2 An electron micrograph of Vibrio cholerae. The
faeces of an infected person are full of these bacteria, each
with its distinctive flagellum (× 13 400).