Science > Biology > Applied Biology > Human Health and Diseases > Communicable or Infectious Diseases
Communicable diseases are those diseases which spread from one person to another through a variety of ways that include: contact with blood and bodily fluids; breathing in an airborne virus; or by being bitten by an insect. They are spread by pathogens like viruses, bacteria, fungi, protozoa, helminths, and certain arthropods, etc.
The possible connection between living organisms and disease (cause of communicable diseases) was first proposed by Italian physician Fracastorius in 1546. He proposed that syphilis is caused by virus Contaginum vivam through live contact. Louis Pasteur and Robert Koch (1876-85) proposed the Germ Theory of Disease (related to communicable diseases). Which states that “The germ theory of disease states that certain diseases are caused by specific germs or infectious agents”.
Koch collected rod-shaped anthrax bacteria from farm animals that had died of the disease and used them to infect healthy mice. He also carried out a control experiment, using exactly the same method but substituting the anthrax bacilli with blood from healthy farm animals. The anthrax-infected mice developed the disease and died, but the control mice remained healthy: Koch now had clear evidence that the anthrax bacteria had caused the disease.
The Reservoir of Infection:
Any person, animal, plant, soil or substance in which an infectious agent normally lives and multiplies is called the reservoir of infection. The reservoir typically harbors the infectious agent without injury to itself and serves as a source from which other individuals can be infected.
The infectious agent primarily depends on the reservoir for its survival. It is from the reservoir that the infectious substance is transmitted to a human or another susceptible host.
The soil is a reservoir of Clostridium tetani. Air is a reservoir of bacterial cysts of tuberculosis, diphtheria, the virus of measles and mumps and other diseases causing bacteria. Food and water are reservoirs for the cyst of cholera, typhoid, Entamoeba histolytica and helminths like Taenia, Ascaris, and Wuchereria, etc. Carriers or vectors are those living organisms that spread pathogens from an infected person to healthy person but themselves are not affected. Vectors are the organisms that transmit disease but itself remains uninfected and disease-free. Carriers are the organisms that transmit disease itself are infected but remain disease-free.
Carriers of Communicable Diseases:
|
Carrier |
Diseases |
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Mosquitoes |
Malaria, lymphatic filariasis, Japanese encephalitis, some haemorrhagic fevers (yellow fever, dengue, dengue haemorrhagic fever) and viral fevers (West Nile). |
|
Tsetse flies |
Sleeping sickness |
|
Fleas |
Plague, murine typhus |
|
Mites |
Scrub typhus, scabies |
|
Houseflies, cockroaches |
Intestinal, skin, and eye infections |
|
sandflies |
Leishmaniasis |
|
Body lice |
Typhus fever, louse-borne relapsing fever |
|
Carrier |
Pathogen |
|
Tsetse flies (Glossina) |
Trypanosoma |
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Sandflies (Phlebotomus) |
Leishmania |
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Female Anopheles |
Plasmodium |
|
Culex and Aedes mosquitoes |
Wuchereria |
How Pathogens Cause Disease?
Different pathogens act in different ways. Some produce a poisonous substance called toxins. While other invade cells or tissues and produce toxin. The disease symptoms appear only after the lapse of some time, during which period the microbes multiply rapidly. The period between the infection and the appearance of signs of a disease is called the incubation period. The symptoms may be fever, headache, nausea, and vomiting in the body of the host.
Bacteria:
Bacteria stick to a certain cell type. They are able to colonize or penetrate the cell’s surface. If the host is immune to the bacteria due to previous exposure or vaccination, these organisms may not be able to cause disease. The person’s immune system detects their presence and eliminates them. Penetrating the cell surface can result in the destruction of the cell or will allow the bacteria to move inside the cell. Once colonized or inside a cell, bacteria often produce toxins: poisonous substances that cause disease.
However, if there is an imbalance in the host (non-immune), the bacteria is virulent. Small numbers are capable of causing illness, If there is exposure to large numbers of microbes that can paralyse or overcome the immune system, and the host is a nutritious target, then disease may result. Bacteria tend to be less tissue-specific and non-discriminatory than viruses and can cause a variety of infections once they have invaded the host.
Bacteria cause pneumonia, tetanus, tuberculosis and cholera.
Viruses:
They must invade a cell (its host) to reproduce. If the host is immune to the virus due to previous exposure or vaccination, these organisms may not be able to cause disease. The person’s immune system detects their presence and eliminates them. However, if there is an imbalance in the host (non-immune), the microbe is virulent. once inside a cell, bacteria often produce toxins: poisonous substances that cause disease. The host cell burst. Thus there is multi-killing of cells. Viruses cause diseases such as common cold, measles, and influenza.
- Lytic Infections: The virus enters a cell, uses the cell’s resources to make copies of itself, and causes the cell to burst, releasing more virus to the organism. e.g. Common Cold, Influenza
- Lysogenic Infections: A virus integrates its DNA into the DNA of the host cell, forcing the host cell to replicate the viral DNA with every regular DNA replication. e.g. HIV, Hepatitis B.
The mass killing of cells can cause a wide range of diseases. The intestinal cells reproduce rapidly any infection of these cells does not last long and no permanent damage is done. e.g. Stomach Flu and Rotavirus
The viral infection of neurons, which cannot be replaced can cause permanent damage including paralysis e.g. Encephalitis and Viral Meningitis
The viral infection of immune system cells can disable a person’s ability to defend itself against pathogens. e.g. Lupus and HIV
Fungi:
Fungi thrive in warm, moist environments. Microscopic fungi normally colonize dead skin and hair. Occasionally the fungus can invade the skin or can be inhaled, causing infection through the release of toxins. e.g. Athlete’s Foot, Jock Itch, Ringworm. Fungi not only produce diseases but they attack food surfaces.
Protists:
Protists commonly invade the bloodstream (Malaria and African Sleeping Sickness) or the intestines (Dysentery). Protists are large enough that the body tries to fight them off, causing illness. Protists can release waste that causes a severe reaction or infection.
Nutrients meant for the host are used by the protist, and any disease related to lack of nutrients may occur. Protozoa cause malaria and amoebic dysentery.
Worms:
Animals generally get worms by ingesting their eggs, which then mature inside the warm, nutrient-rich body. Worms latch onto a tissue and consume cells and nutrients intended for the host, often leading to nutrient deficiency. Some worms burrow into tissue and cause infection in the host.
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