Communicable Diseases

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In today’s class, we will be talking about communicable diseases. Enjoy the class!

Communicable Diseases

Disease

Disease means illness or disorder of the body. It is also referred to as an abnormal condition that affects the parts or whole of a living organism. Diseases are caused by pathogens: bacteria, rickettsiae, spirochetes, viruses, fungi, protozoa, and worm. Diseases are always accompanied with specific symptoms and signs. However, symptoms can be used to figure out what kind of disease somebody is suffering from. Examples of diseases include Ebola, Syphilis, Cholera, Dysentery, Gonorrhea, AIDS, Cancer, Asthma, Hypertension, Diabetes, Anaemia, etc. Some diseases, communicable diseases, can be transferred from one person to the other. However, some diseases, non-communicable diseases, cannot be transferred to another.

Communicable diseases are diseases that are transmitted from a disease carrier or reservoir through direct contact with an infected individual or indirectly through a vector (disease-transmitting organism or parasite). The spread often happens via airborne viruses or bacteria, but also through blood or other bodily fluid. Examples of the communicable disease include anthrax, dysentery, typhoid fever, Lassa fever, Ebola virus disease, chickenpox, measles, mumps, pneumonia, whooping cough, meningitis, tuberculosis, gonorrhoea, syphilis, HIV/AIDS, yellow fever, leprosy, cold, poliomyelitis, tetanus, rubella, tinea (ringworm), rabies, hepatitis, etc. Communicable diseases are classified into nine types according to the nature of the pathogen( causing agent). These are:

1. Viral diseases: These include polio/ poliomyelitis, rabies, viral hepatitis/ jaundice, chikungunya, dengue fever, common cold/rhinitis
2. Rickettsial diseases: These are caused by rickettsiae; the obligate intracellular parasites. Examples include rocky mountain spotted fever, rickettsialpox, trench fever, fever and epidemic typhus fever.
3. Mycoplasmal diseases: These are the smallest free-living microorganisms. They can produce filaments, which resemble fungi.
4. Chlamydial diseases: Chlamydia is also microorganisms that are intracellular parasites.
5. Bacterial diseases: These include typhoid/enteric fever, pneumonia, tuberculosis, cholera, etc.
6. Spirochaetal diseases: These are flexible, twisted, round and long axis bacterium, which are responsible for syphilis.
7. Protozoan diseases: These include diseases such as malaria, amoebiasis, diarrhoea, vaginitis, balantidium dysentery, dum-dum fever.
8. Helminthic diseases: These include ascariasis, filariasis/elephantiasis, etc.
9. Fungal diseases: These include ringworm and athletes foot.

Pathogens

Pathogens are anything that can produce disease. The term is used to describe infectious agents such as bacteria, virus or other small (micro) organisms that cause disease on its host, which may be an animal, plant, or other microorganisms. Pathogens cause different types of diseases in humans such as measles and whooping cough, and these diseases, in turn, affect performances.

Mode of transmission of diseases

This refers to any mechanism by which an infectious agent is spread from a reservoir to a person. The mode of transmission of diseases is categorized mainly as either direct or indirect. Once an infectious agent leaves a reservoir or carrier, it gets transmitted to a new host, multiplying and causing disease. The route by which an infectious agent is transmitted from a reservoir to another host is called the mode of transmission.

Direct mode of transmission

Direct transmission refers to the transfer of an infectious agent from an infected host or reservoir to a new host, without the need for intermediates such as air, food, water or other animals. Subtypes include:

1. Direct contact: Touching, biting, kissing, and sexual intercourse. Diseases include sexually transmitted diseases (STDs), human immunodeficiency virus (HIV), syphilis, etc.
2. Direct projection of droplet: Spray into the eye, nose, or mouth during sneezing, coughing, spitting, singing, or talking. Diseases include common cold/rhinitis, influenza, all mycoplasma diseases, etc.
3. Animal bite transmission: such as mad dogs, cats, wolves, Aedes aegypti mosquito, etc. Diseases include rabies, dengue fever, chikungunya, etc.
4. Placental transmission: This refers to transmission of infectious agent usually from a pregnant woman to her foetus through the placenta. It is known as Mother-To-Child – Transmission (MTCT). A major example is HIV.

Indirect mode of transmission

This is when infectious agents are transmitted to new hosts through intermediates such as air, food, water, objects/substances in the environment, or other animals. Subtypes include:

1. Airborne transmission: The infectious agent is transmitted in dried secretions from the respiratory tract, which can remain suspended in the air for some time. E.g. tuberculosis.
2. Vehicle-borne transmission: A vehicle is any non-living substance or object that can be contaminated by an infectious agent, which then transmits it to a new host, e.g. food, water, milk, plasma, etc. Diseases include cholera Typhoid, Pneumonia, Scarlet fever, etc.
3. Vector-borne transmission: A vector is an organism, usually an arthropod, which transmits an infectious agent to a new host. Arthropods, which act as vectors include houseflies mosquitoes, lice and ticks. Disease include Dum-Dum fever, Elephantiasis Trichomoniasis, Amoebiasis, Malaria Diarrhoea, etc.

Factors and causative agents

Disease transmission is a process in which several events happen one after the other in the form of a chain. Some factors aid and are involved in the complete transmission of diseases from a reservoir to a new susceptible host. They include:

1. Infectious or causative agent: these are microorganisms that cause diseases.
2. Reservoir: Also known as infected: these hosts are disease-carrying substances. They include living things like humans and animals, or nonliving things like soil, food, water and the environment. Nonliving reservoirs are called vehicles.
3. Route of exit: This is the site through which an infectious agent leaves an infected host to be transmitted to the environment or other persons. E.g. respiratory tract, gastrointestinal tract, the skin, etc.
4. Mode of transmission: This is the route by which an infectious agent is transmitted from a reservoir to another host.
5. Route of entry: This is the site through which an infectious agent enters a host from a reservoir. E.g. the skin, respiratory tract, Gastrointestinal tract, etc.
6. Susceptible host: This is an individual who is likely to develop a communicable disease after exposure to the infectious agents.

Causative agents

This refers to the biological pathogen that causes a disease. They include:

• Virus
• Parasite
• Fungus
• Bacterium

Technically, the term can also refer to a toxin or toxic chemical, and other organisms like prions, protozoans and worms that cause illness.

 

In our next class, we will be talking more about Communicable Diseases.  We hope you enjoyed the class.

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