Infectious Diseases and Defence System of the Human Organism

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Brief Introduction

An infectious disease is an impairment to normal tissue functions that is caused by the invasion of a host by agents who through their activities harm the host’s tissues, and it can be transmitted to another person i.e. infectious. The difference between an infection and a disease is that an infection results when a pathogen enters a host’s body and begins to grow in it whereas it only becomes a disease if there is tissue damage as a result of the pathogen invading and growing in a host. Pathogens are microorganisms that cause diseases. It is worth noting that not all microorganisms are pathogens as some fight the pathogens for resources in the body thereby inadvertently providing an immune system to the body.

A true pathogen can cause a disease in almost any vulnerable host regardless of the presence of an immunosuppression or lack of it. Opportunistic pathogens on the hand are probably infectious agents who are only activated when there is a compromise in the body’s immune system. Bodies with weaker immune systems can be like those of the elderly, cancer patients undergoing chemotherapy which usually affects the immune system and people living with HIV/AIDS. The severity of diseases caused by a true pathogen varies in form from mild to extreme while the diseases associated with opportunistic pathogens are almost always severe. Some infectious pathogens can be transmitted easily i.e. they are very contagious but they rarely diseases. An example is the Polio virus which infects most people who contact it but only manifest in about 5% of these people in the form of a clinical disease… There are infectious agents that cause diseases easily but are not easily contracted. These agents are said of to be virulent but less contagious. An example of this type is the Ebola Hemorrhagic Fever which has close to a 90% mortality rate but which can not be contracted easily via contact. Finally, there is the dangerous group of very virulent and easily contagious infectious pathogens.

Organisms Responsible for Infectious Diseases

There are five different types of microbes that cause infections. These are:


These are single-cell microorganisms with no organized internal membrane structures like nuclei, mitochondria et al. a system referred to as being prokaryotic. They have a circular, double-stranded DNA genome with little protein. There reproduction process involves growing in size and then dividing into two different organisms, a process also known as binary fission. Examples of disease-causing bacteria are: Mycobacterium Tuberculosis – which causes tuberculosis. Tuberculosis has a high disease burden, that is to say, the financial costs of controlling, treating and eradicating it are very high as well as its mortality and morbidity rate. It kills close to 2 milion people annually in sub-Saharan Africa. Chlamydia – a phylum of parasites that only grown in cells and can cause pneumonia, Urinary Tract Infections (UTIs) and may be involved in Ischemic Heart Disease (IHD) which causes a reduction in blood flow to the coronary muscles. Neisseria meningitidis, Streptococcus pneumoniae, and Haemophilus influenzae type B bacteria all which cause different kinds of bacterial meningitis. Cellulitis, Folliculitis, and Impetigo which cause skin infections like red, swollen bumps, oozing pores, boils, and painful red infections on the skin.


Viruses are sometimes considered as not being organisms because except for the host cell, they lack metabolism and cannot reproduce on their own. To reproduce, viruses infect a host then use the host’s DNA repair and replicate functionality to create copies of themselves. A virus has a genome of nucleic acid that is surrounded by a protein coat (unlike bacteria). The genome may be double or single-stranded DNA in a DNA virus and Single or double-stranded RNA for an RNA virus. A DNA virus infects a host by first attaching itself to a specific host cell using its protein receptors or the capsid. RNA viruses can use the genetic material as a direct messenger in order to produce viral proteins that copies the RNA template to create a genetic substance for new viral particles. Examples of pathogenic viruses are: As is the case with other pathogens, a virus causes diseases by damaging normal cell system… They do this by creating repressor proteins that prevents the manufacture of the host cell’s proteins.

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Examples of DNA viruses are the poxvirus which causes smallpox and the herpes virus which is the causative agent for numerous diseases like painful genital parts, cold sores, and chickenpox. Common examples of RNA viruses are myxoviruses and paramyxoviruses which cause influenza, mumps and measles, rhinoviruses that are responsible for common cold, rotaviruses responsible for gastroenteritis, and the retroviruses which cause AIDS and the different types of cancer. Fungi Unlike bacteria, fungi are eukaryotic and mostly multi-cellular (except for yeast) but they cannot manufacture their own food so they depend on other organisms and get nutrients from them. They have a rigid cellulose and their reproduction process involves forming spores. Example fungal diseases are histoplasmosis which is a lung infection caused by bird droppings, and ringworms. There are also yeast of the Candida genus that causes vaginal yeast infections and throat infections.


Protozoa, just like fungi are heterotrophic but unlike fungi, they are unicellular. Examples of protozoa are amoeba and paramecium. Protozoa may be transmitted through contaminated food, water or by an infected arthropod like a mosquito. Giardia lamblia and Cryptosporidium parvum are a species of protozoa that cause diarrhea. Malaria, a prevalent disease in tropical conditions that registers almost 500 million cases annually, is caused by Plasmodium.


These are multicellular invertebrate animals with differentiated tissues of which some are infectious parasites… Parasitic helminthic diseases tend to be difficult to cure as drugs that kill them are also destructive to human cells since helminths and humans are both animals thereby making them have almost similar physiologies. Trichinosis is a disease caused by a helminth known as Trichinella spiralis. This agent is usually contracted through the ingestion of improperly cooked pork from infected pigs. Bilharzia (also known as schistosomiasis) is a disease frequent in Africa and Latin America caused by Schistosoma, another example of a helminth.

Ways in Which Pathogens Cause Diseases

As outlined earlier, infections do not always lead to a disease and it only becomes a disease when the cells in the body are damaged due to an infection. Infectious microbes challenge the body’s immune system in a varied number of ways. A viral infection, for example, causes diseases by killing cells or destroying the cell functions. The body responds by generating heat which most viruses cannot survive in (which is the fever that one feels), secreting the interferon chemical that blocks the reproduction of viruses or by commanding antibodies to attack the intruder.

How Diseases Can Be Transmitted via Vectors and How They Enter the Body

Vectors are living things that can spread diseases among humans or from animals to humans. Vector-borne diseases are illnesses spread through the bite of infected arthropod species like mosquitoes, sandflies, and triatomine bugs. Vector-borne diseases make up 15% of all infectious diseases resulting in approximately 800000 deaths yearly.

These arthropods are cold-blooded and are therefore high affected by weather changes. Weather determines the survival and growth rate of vectors which in turn influences how suitable a habitat can be and the abundance of the vectors in a particular place. Other factors that influence vector growth include pesticide application, land use, habitat destruction, and host density… Pathogenic microbes enter the human body through the mouth, nose, eyes, urogenital openings or through vector bites that break the skin barrier. Some infections are transmitted through direct contact with the infected skin, body fluids or mucous membranes. Some of the diseases transmitted like this include cold sores and sexually transmitted diseases like AIDS.

Other diseases can spread via indirect touch when an infected person touches a surface such as a countertop thereby leaving behind pathogens and then a healthy individual touches the same surface before it is cleaned. Sometimes, coughing, talking or sneezing by a patient can spread droplets of microbes that transmit the disease to a healthy person when the microbes get in contact with the mucous membrane of his or her mouth, eyes or nose. Examples of airborne diseases include measles, Legionnaires’ disease, tuberculosis, and hantavirus pulmonary syndrome. Vectors can also be transmitted via vehicles like dirty water, contaminated food, or blood. Microbes like E. coli and salmonella bacteria are usually transmitted into the digestive system like this. The most prevalent vector that spreads human infections is mosquitoes. Mosquitoes have been known to transmit malaria, Zika, dengue fever, yellow fever, chikungunya, and the West Nile Virus. Other vectors include mites, ticks, and fleas et al.

Non-Specific Defence System

Non-specific defence systems are known so because they do not attack a specific pathogen but rather work in general. The skin is the first non-specific defense system since it acts as an almost impenetrable barrier to microbes. The skin is fortified with chemicals, oil, and sweat which renders the surface of the skin acidic and hence inhabitable to most pathogens. Enzyme lysozyme found in sweat digests the walls of most bacteria. Mucous membranes are coatings of epithelial tissue that release mucous, a sticky viscous liquid that covers most surfaces that are vulnerable to attacks from pathogens. Mucus is secreted in nasal passages, respiratory passages, digestive systems, and the reproduction system where it serves as a protection against pathogens. Mucous membranes together with the skin is known as the first line of defence and are easily penetrable by trauma, increased permeability or viral multiplication in the endothelial tissues.

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