Communicable diseases:
Epidemiology surveillance and response
Definitions
A communicable (or infectious) disease is one caused by transmission of a specific
pathogenic agent to a susceptible host. Infectious agents may be transmitted to
humans either:
� directly, from other infected humans or animals, or
� indirectly, through vectors, airborne particles or vehicles.
Vectors are insects or other animals that carry the infectious agent from person to
person. Vehicles are contaminated objects or elements of the environment (such as
clothes, cutlery, water, milk, food, blood, plasma, parenteral solutions or
surgical
instruments).
Contagious diseases are those that can be spread (contagious literally means “by
touch”) between humans without an intervening vector or vehicle. Malaria is
therefore
a communicable but not a contagious disease, while measles and syphilis are
both communicable and contagious. Some pathogens cause disease not only through
infection but through the toxic effect of chemical compounds that these produce.
For example, Staphylococcus aureus is a bacteria that can infect humans directly,
but
staphylococcal food poisoning is caused by ingestion of food contaminated with a
toxin that the bacteria produces.
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Role of epidemiology
Epidemiology developed from the study of outbreaks of
communicable disease and of the interaction between
agents, hosts, vectors and reservoirs. The ability to describe
the circumstances that tend to spark epidemics in
human populations – war, migration, famine and natural
disasters – has increased human ability to control the
spread of communicable disease through surveillance,
prevention, quarantine and treatment.
The burden of communicable
disease
The estimated global burden of communicable diseases –
dominated by HIV/AIDS, tuberculosis and malaria – is
shown in Box 7.1. Emerging diseases such as viral haemorrhagic
fevers, new variant Creutzfeld-Jakob disease and
severe acute respiratory syndrome (SARS), as well as reemerging
diseases including diphtheria, yellow fever,
anthrax, plague, dengue and influenza place a large and
unpredictable burden on health systems, particularly in
low-income countries.
Threats to human security and
health systems
Communicable diseases pose an acute threat to individual
health and have the potential to threaten collective human
security. While low-income countries continue to deal
with the problems of communicable diseases, deaths due
to chronic diseases are rapidly increasing, especially in
urban settings . Although high-income
countries have proportionally less communicable disease
mortality, these countries still bear the costs of high morbidity
from certain communicable diseases. For example,
in high-income countries, upper respiratory tract infections
cause significant mortality only at the extremes of
age (in children and elderly people). However, the associated
morbidity is substantial, and affects all age-groups
Using epidemiological methods to investigate and
control communicable disease is still a challenge for the
health profession. Investigation must be done quickly and
often with limited resources. The consequences of a successful
investigation are rewarding, but failure to act effectively can be damaging. In the
AIDS pandemic, 25 years of pidemiological studies
have helped to characterize the agent, modes of transmission and effective means of
prevention. However, despite this knowledge, the estimated global prevalence of HIV
in 2006 was 38.6 million cases, with 3 million deaths per year.
Epidemic and endemic disease
Epidemics
Epidemics are defined as the occurrence of cases in excess of what is normally
expected in a community or region. When describing an epidemic, the time period,
geographical region and particulars of the population in which the cases occur must
be specified.
The number of cases needed to define an epidemic
varies according to the agent, the size, type and susceptibility
of population exposed, and the time and place of
occurrence. The identification of an epidemic also depends
on the usual frequency of the disease in the area
among the specified population during the same season
of the year. A very small number of cases of a disease not
previously recognized in an area, but associated in time
and place, may be sufficient to constitute an epidemic. For
example, the first report on the syndrome that became
known as AIDS concerned only four cases of Pneumocystis
carinii pneumonia in young homosexual men.3 Previously
this disease had been seen only in patients with
compromised immune systems. The rapid development of
the epidemic of Kaposi sarcoma, another manifestation of
AIDS, in New York is shown in Figure 7.3: 2 cases occurred
in 1977 and 1978 and by 1982 there were 88 cases. 4
The dynamic of an epidemic is determined by the
characteristics of its agent and its pattern of transmission,
and by the susceptibility of its human hosts. The three
main groups of pathogenic agents act very differently in this respect. A limited
number
of bacteria, viruses and parasites cause most epidemics, and a thorough
understanding
of their biology has improved specific prevention measures. Vaccines, the most
effective means of preventing infectious diseases, have been developed so far only
for some viral and bacterial diseases. If the attempt to make a malaria vaccine is
successful, this will be the first vaccine for a parasitic disease. Vaccines work
on both
an individual basis, by preventing or attenuating clinical disease in a person
exposed
to the pathogen, and also on a population basis, by affecting herd immunity