Introduction to communicable disease

Communicable disease
Common Terms
• Communicable diseases: Are those which can be transmitted from one person to another or from animal to person.
• Epidemic (from Greek epi- upon + demos people) :
A classification of a disease that appears as new cases in a given human population, during a given period, at a rate that substantially exceeds what is ‘expected’, based on recent experience.
o Therefore it is the unusual occurrence of a disease in the community

• Endemic: The constant presence of a disease or infectious agents within a community
• Pandemic (from Greek pa? pan all + demos people): An epidemic that spreads across a large region (for example a continent), or even worldwide.
• Nosocomial: An infection developing in a patient while in a hospital or acquired in a hospital but could show up after discharge.

• Principle of disease transmission: Transmission of a disease in human requires the following components:
o An agent capable of infecting man
o A source: an infected host or reservoir of infection
o A portal of exit from the source
o A suitable means of transmission
o A portal of entry into a new host
o A susceptible host

• Agent: An agent is an organism, mainly a microorganism but including helminthes that is capable of causing a disease.
• Reservoir (of infection): Refers to any human beings, animals, arthropods, plants, soil or inanimate matter or a combination of these in which an infectious agent normally lives and multiplies, and on which it primarily depends for survival and reproduction in such a manner that it can be transmitted to a susceptible host.
o A reservoir of infection can also be termed as the natural habitat of the infectious agent.
o Disease that man is the only reservoir is called anthroponeses e.g. Measles and cholera.
o Those that involve other animals reservoirs are called zoonoses e.g. plague and rabies.
• Portal of exit in the human host: Include the respiratory passages, the alimentary canal, the opening in the genital urinary system and the skin lesion
• Suitable means of transmission: Transmission of infectious agent is any mechanism by which a susceptible host is exposed to an infectious agent. It may be either direct or indirect.

Medical Importance of Communicable Diseases
• Many of them are very common
• Some of them are serious and cause death and disabilities
• Some of them cause wide spread outbreak of disease that is epidemic
• Most of them are preventable by fairly simple means
• Many are serious particularly in infants and children

Groups of Communicable Diseases
Contagious disease
• Contagious (contact diseases) are diseases transmitted by direct or indirect contact.
o Direct contract is by skin to skin e.g. touching an infected person.
• Indirect contact is by handling contaminated objects such as clothing, bedding materials, dressing and utensils.

Sexual Transmitted Infections and HIV/AIDS
• These are diseases or infections whose predominant mode of transmission is through sexual contact, be it heterosexual or homosexual.
• Sexually transmitted infection includes the stage of preclinical illness (i.e. the typical symptoms and signs have not yet appeared) while sexually transmitted disease includes the stage of clinical illness (i.e. the typical symptoms and signs are present).

Vector-Borne Diseases
• Vectors are invertebrate hosts (insects, ticks and snails) which are an essential part of the life cycle of the disease causative organism.
• Therefore a vector-borne is a disease whose transmission requires a vector (i.e. part of the life cycle of the causative organism takes place within the vector).
• They are taken for development of the organism outside the human body is called extrinsic incubation period.
• Note that a housefly that carries organisms on its body and contaminates food is not regarded as a vector because it simply mechanically transmits the organism causing the disease.

• Vectors acquire disease organisms by sucking blood from infected persons or animals and pass them on by same route.
• Note that infection may enter skin cracks or abrasions either from infected insect faeces deposited when feeding or from body fluid when an insect is crushed.

Diseases Caused by Faecal Contamination
• These are diseases whose causative organisms are excreted in the stools of infected persons or animal.
• The portal of entry for these organisms is the mouth.
• The organisms have to pass through the environment from the faeces of an infected person or animal to the gastrointestinal tract of a susceptible person (i.e. faecal – oral transmission route).
• Very small quantities of faeces can carry enough organisms to establish infection.
• Contaminated food may smell, look and taste normal and yet harbour pathogenic
organisms.
• Hands that appear clean may carry and transmit enough micro-organisms to spread diseases.
• Food plays a central role in transmitting disease as it can be directly or indirectly
contaminated via polluted water, dirty hands, contaminated soil, flies and animals or animal products.
• Water can be polluted directly by faeces, or faecal material may be washed in from polluted soil.
• Hands are contaminated after defaecation or by touching contaminated objects.

Modes of Transmission of Communicable Diseases
• Direct or indirect contact
• Sexual Contact
• Vector bite, crushed or infection enter skin cracks/abrasions from infected insect
• Faecal-oral route
• Penetration through intact skin
• Ingestion of infected animal products
• Inhalation (droplet infection)

Principles of Communicable Diseases Prevention and Control
• Communicable diseases transmission can be controlled by three main ways namely
o Attacking the source
o Interrupting transmission
o Protecting the host

Level of Prevention
• Primary prevention – these refers to prevention of healthy people from becoming infected and or develop a disease.
• This includes:
o Immunization
o health education
o promotion of nutrition
o vector control
o provision of adequate housing
o Hand washing

• Secondary prevention - This refers to detection of people who already have a given disease at the earliest possible stage in order to stop the disease from developing further and or prevent complications i.e. early detection and prompt treatment.
• Tertiary prevention – This refers to prevention of disease disability and death in a patient who has already developed the disease, and has complications or cannot be cured.
• This includes Rehabilitation

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