Communicable diseases can be spread directly from person to person through contact or droplets, or indirectly through vectors like mosquitoes or vehicles like contaminated food or water. The key to preventing transmission is breaking the cycle by treating infected individuals, improving hygiene to interrupt transmission routes, or protecting susceptible hosts through vaccination or other preventative measures. Some major global communicable diseases that cause millions of deaths each year spread through respiratory droplets or airborne routes. Controlling communicable diseases requires understanding how the infectious agent, host, and environment interact and taking measures to alter the balance against the agent.

1. BY
KWIZERA DENIS (MBChB)

2. Communicable Diseases
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.
(WHO, 2006)
 There are many examples of communicable diseases, some of
which require reporting to appropriate health departments
or government agencies in the locality of the outbreak.
 Some examples of the communicable disease include HIV,
hepatitis A, B and C, measles, salmonella, and blood-borne
illnesses…More examples have been linked to the content
of this presentation as shall be elaborated!

3. (2)
They are called communicable because they are able to
spread from one living animal to another, such as man to
man, animal to man, or animal to animal. In this situation
the man or animal is called a host.
These communicable diseases are all caused by some living
organisms or agents, such as a vims, rickettsia,
mycoplasma, bacteria, viruses, protozoan, helminthes, or
insects.
 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.

4. Global burden of communicable diseases
 Communicable diseases account for 14.2 million deaths each year. Another 3.3
million deaths are attributable to maternal and perinatal conditions and
nutritional deficiencies. Together these account for 30% of the world’s deaths
(WHO, 2006)
 Six causes account for almost half of all premature deaths, mostly in children and
young adults, and account for almost 80% of all deaths from infectious diseases:
 Acute respiratory infections (3.76 million)
 HIV/AIDS (2.8 million)
 Diarrhoeal diseases (1.7 million)
 Tuberculosis (1.6 million)
 Malaria (1 million)
 Measles (0.8 million)
Most of these deaths occur in low-income countries.
 WHO projections suggest that – due to better prevention – total deaths from these
causes will decline by 3% over the next 10 years.

5. Disease cycle
 With these diseases we need to
consider the living agent, the
host it infects, and the
environment that both live in.
 To practice the control of
epidemics or diseases effectively
it is necessary to understand this
balance between host, agent, and
environment
 And what practical, simple, and
cheap methods can be
undertaken to alter the balance
against the agent? since these
three factors are the
determinants of infection
spread.
Host
EnvironmentAgent

6. Transmission cycle
infectious
agent
reservoir
portal of exit
mode of
transmission
portal of
entry into
host
susceptible
host
• In order to contract or
spread an infectious
disease each link of this
chain must be intact.
• If the chain is broken at
any point than the spread
of the infection is stopped.
This is true of all infectious
disease eg HIV-AIDS,
Hepatitis, C- Diff,
Influenza, TB, etc

7. Routes of transmission
 The pathway of causative agents from a source to
infection of a susceptible host is called 'transmission
route'.
 The characteristic of the transmission route depends
mainly on the characteristics of the causative agent
and those of the host
 Some micro organisms are restricted to a limited
number of transition routes, whereas others can follow
many different pathways to infect their hosts

8. (2)
 It is useful to have detailed knowledge about the specific
transmission routes of pathogens, since this gives practical
information of effective control measures by interrupting the
spread of the infection within the population.
 Each disease organism has particular routes and these
therefore play a large part in how these organisms spread in
the community. For example, some are spread in water and
food and others by vectors like mosquitoes and snails.
 The main routes are by:
 Airborne droplets
 Contact with animals or their products.
 Direct contact
 Faecal contamination of soil, food, and water
 Vectors

9. (3)
Modes of transmission
An infectious agent may be transmitted from its natural
reservoir to a susceptible host in different ways. There are
different classifications for modes of transmission. Here is
one classification:
Direct
Direct contact
Droplet spread
Indirect
Airborne
Vehicle borne
Vector borne (mechanical or biological)
In direct transmission, an infectious agent is transferred from
a reservoir to a susceptible host by direct contact or droplet
spread.

10. Direct transmission
 This means direct and immediate transfer of infectious
agents to a susceptible host.
 This may be through direct contact such as touching,
biting, kissing or sexual intercourse, or by the direct
projection of droplet (droplet spread) spraying onto eyes,
nose or mouth of other people during sneezing, coughing,
spitting, singing or talking. Droplet spread is usually
limited to short distances, such as 1 meter or less).
 Direct transmission routes are linked to behavior, and most
interventions that target this particular transmission
usually aim to educate people to reduce risk behavior (e.g.
condom use, using facial masks while contacting patients,
sneeze in handkerchiefs or sleeves, etc)

11. (2)
 Direct contact occurs through skin-to-skin contact,
kissing, and sexual intercourse.
 Direct contact also refers to contact with soil or
vegetation harboring infectious organisms. Thus,
infectious mononucleosis (“kissing disease”) and
gonorrhea are spread from person to person by direct
contact. Hookworm is spread by direct contact with
contaminated soil.

12. (3)
 Droplet spread refers to spray with relatively large,
short-range aerosols produced by sneezing, coughing,
or even talking.
 Droplet spread is classified as direct because
transmission is by direct spray over a few feet, before
the droplets fall to the ground. Pertussis and
meningococcal infection are examples of diseases
transmitted from an infectious patient to a susceptible
host by droplet spread.

13. Indirect transmission
 When transmission of infectious organisms occurs
from a source through objects (vehicles) or insects
(vectors) we call this indirect transmission.
 Transmission through vehicles is usually linked to
processes, such as food production, food handling,
cleaning procedures in day care centers, hygiene
procedures in medical facilities etc.

14. Vertical transmission
 A specific form of direct transmission is that between
mother and child during pregnancy or childbirth.

15. Routes of Transmission
Airborne transmission
 Airborne transmission occurs when infectious agents are carried by dust suspended
in the air.
 With airborne transmission, direct contact is not needed to spread disease (as
compared with respiratory droplet transmission).
 Measles (rubeola)
 Tuberculosis (TB)
Respiratory (droplet) transmission
 Some disease-causing bacteria and viruses are carried in the mouth, nose, throat and
respiratory tree.
 They can spread by coming into direct contact with droplets when an infected person
coughs or sneezes, or through saliva or mucus on unwashed hands.
 Chickenpox (varicella)
 Influenza (flu)
 Measles (rubeola)
 Pertussis (whooping cough)
 Respiratory Syncytial Virus (RSV)
 Tuberculosis (TB)

16. Control and prevention of communicable
diseases
 Direct cause is interrupted by preventing contact with
source.
 Indirect cause require different approaches such as
provision of mosquito nets, adequate ventilation, cold
storage for foods, sterile syringes and needles in case of
blood transfusion, sanitation

17. PRINCIPLES OF COMMUNICABLE DISEASE CONTROL
The aim is to lower the incidence of the disease to a level
that is no longer a problem to the community.
When a disease is under control, the control measures
normally have to be continued indefinitely, since the
incidence may start to rise again if they are stopped.
Ideally, we would like to eradicate all communicable
diseases, but in practice this is only occasionally possible,
e.g. smallpox.
The methods used to turn the ecological 'balance' against
the agent by attempting to break the transmission cycle
operate at one of the three points by:
 Attacking the source
 Interrupting the route of transmission
 Protecting the susceptible host.

18. (2)
Attacking source Interrupting
transmission
Protecting susceptible
host
 Treatment of cases
and carriers
 Isolation
 Surveillance of
suspects
 Reservoir control
 Notification
 Environmental
hygiene
 Personal
hygiene
 Vector control
 Disinfection
and
sterilization
 Population
movements
 Immunization
 Chemoprophylaxis
 Personal protection
 Better nutrition

19. Prevention and control levels
Primary prevention is achieved by all the methods listed
under 'interrupting transmission' and under 'protecting
the susceptible host', together with control of animal
reservoirs. If all these arc properly carried out the number
of new cases could be greatly reduced, e.g. clean water
supplies and the correct disposal of faeces could stop a lot
of gastroenteritis, anophelese mosquito control could stop
malaria transmission, and immunization with BCG and
measles vaccines could protect most young children.
Secondary prevention can be achieved by finding
subclinicalcases and carriers and by tracing and
surveillance of contacts.
Tertiary prevention is by the treatment of cases so that
they do not spread the infection any further.

20. Attacking the source
Treatment of cases
 If sufficient clinical cases can be treated with
chemotherapeutic drugs that are effective against the
organism, then these organisms cannot spread to new
hosts, e.g. in tuberculosis and leprosy.
 This is called mass treatment and its effectiveness
depends on the coverage that can be obtained over all
the infective cases in the community.
Good to note down:
Clinical infection
Subclinical infection

21. (2)
Subclinical cases and carriers
 The same applies to subclinical cases and carriers as to the
treatment of clinical cases. But with these patients special
efforts have to be made to find them first, as they do not
usually present with any apparent illness, e.g. subclinical
infectious hepatitis, or ankylostomiasis.
 The most important method for finding subclinical cases is
through contact tracing. This means going to each clinical
case, getting from him the names of all his contacts,
finding these people and doing something about their
exposure (testing, surveillance, prophylaxis, etc).
 In addition to contact tracing, screening methods and
surveys may have to be used…………….CONTACT TRACING
IS AN IMPORTANT PART OF SECONDARY PREVENTION

22. (3)
Isolation of cases
 Isolation means that the patient is not allowed to come
into close contact with other people, so that the organisms
cannot spread. Isolation is very difficult to enforce but was
very successfully used in the eradication of smallpox.
Surveillance of contacts
 If a susceptible host has been exposed to a case or sources
of infection it may be necessary to keep him under close
watch and out of contact with other people for the time of
the maximum incubation period. This particularly applies
to contagious diseases like plague. This form of control
used to be called quarantine……..CASES ARE USUALLY
THE MAIN SOURCES OF INFECTION.

23. (4)
Reservoir control
 In those diseases that have their main reservoir in animals,
mass treatment, chemoprophylaxis, or immunization can
be used, e.g. trypanosomiasis and brucellosis. Other ways
include separating man from animals or killing the animals
and so destroying the reservoir, e.g. plague and rabies.
Notifications and reports
 Although these do not directly affect the source,
notifications are an essential means of keeping a watch
(surveillance) on the number of new cases and thereby
monitoring the effectiveness of the control programme.
 Notifiable diseases and epidemics should be reported to
the Ministry of Health via the DMO. A good notification
system provides early warning of epidemics before they
become serious.

24. Interrupting transmission
Environmental hygiene
 Many organisms are able to spread through contaminated food and
water, particularly those that are dependant on the faecal-oral route.
 Other diseases are spread through refuse and dirty living conditions.
The airborne diseases are more likely to spread when housing is
inadequate and people live and sleep in crowded rooms
Personal hygiene
 Many personal habits make some diseases more likely, particularly the
contact and venereal diseases and those that may spread due to faecal
contamination of hands, food, and water.
 This is why it is so important to teach children to wash their hands after
using the latrine and before meals, until this becomes an automatic
habit.
Disinfection and sterilization
 These measures aim at destroying the organism when it is in the
environment, e.g. sterilization of surgical instruments to prevent
clostridial and other infections, the chlorinating of water supplies to
prevent typhoid and cholera.

25. (2)
Population movements
 Communicable diseases can be spread by people who are
incubating the illness, by carriers or by actual cases travelling
around.
 During an epidemic it may be necessary to stop people moving
around or going on safari, and even to forbid gatherings like
markets or festivals whilst the epidemic lasts. Migration of people
and refugees can spread diseases from one area to another.
Vector control
 Any organism that requires a vector, like a mosquito or snail, for
its transmission cycle may be controlled if the vectors can be
killed off or reduced.
 Methods of vector control can be through altering the
environment so that it is unfavourable to the vector (e.g. draining
swamps), by using toxic substances (e.g. larvicides or
molluscicides), or by using other living organisms that attack the
vector (biological methods).

26. Protecting the host
Immunization
 By giving vaccines (made of toxoids, or living or dead
organisms) the level of active immunity can be raised eg,
DPT, BCG, polio, and measles. All these offer personal
protection. If immunization is to be effective in community
control, the population coverage of susceptible has to be
high.
 The protective effect that is obtained when a high
proportion of the population have been immunized is
called herd immunity.
 Passive immunity produced by immune globulins may give
personal protection, e.g. in rabies, but it is not helpful in
mass control…………..IMMUNIZATION GIVES PRIMARY
PROTECTION

27. (2)
Chemopropylaxis
 Drugs that protect the host may be used for suppressing
malaria, and for preventing infection with such diseases as
plaque and cerebrospinal meningitis
Personal protection
 This means some barriers.g. shoes against ankylostomiasis,
nets and insect repellants against mosquitoes.
Better nutrition
 When famine is present then epidemics are more likely to
occur
 Malnourished children also appear more prone to
infections and may suffer from complications such as
measles, and malnutrition. Therefore, the prevention of
malnutrition can help in control of communicable diseases

28. REFERENCES
 Jamison DT, Breman JG, Measham AR, Alleyne G, Claeson
M, Evans DB, et al. (eds).Disease control priorities in
developing countries. New York, Oxford University Press,
2006.
 Adopted from African Medical and Research Foundation
(Community Health)
 Centers for Disease Control And Prevention
 Report on infectious diseases: removing obstacles to health
development. Geneva, World Health Organization, 2005.
 Heymann D. Infectious Diseases. In: Detels R, McEwen J,
Beaglehole R, Tanaka
 K. Oxford Textbook of Public Health. Oxford, Oxford
University Press, 2005.
 (R. Bonita et al, 2006) Basic epidemiology, World Health
Organization, Geneva 2006.