The human body is exposed to a variety of pathogenic organisms including viruses, bacteria, fungi, and parasites . Besides these infectious agents, several toxins either produced by the infectious agents or already present in nature have a deleterious effect on human health [2, 3].
In describing the effect of infectious agents on various components of the human body like cells, tissues, organs, it is important to take into consideration the bodily fluid blood that assures a supply of nutrients and required oxygen almost to every cell of the body and remove toxic waste produced by cellular activity . Besides blood, the lymph, another fluid is mainly present in the spaces between the tissues and in conjunction with the blood protect the human body from various types of infectious agents attack besides curtailing their undesired growth .
Types of infections
This article gives a general overview of infectious agents’ impact on various human body tissues and the fluids around them.
1. Viral Infections
Among the four major groups of infectious agents, viruses are quite unique as they cannot exist in the absence of cells. For a virus to propagate and spread infection, it has to infect a cell and makes its further progeny . There is an extensive list of viruses causing infections among humans like common cold to life threatening EBOLA and acquired immunodeficiency syndrome (AIDS).
Few intriguing facts about viruses are:
i) they are nonliving and need a cell that has to be infected with virus (called permissive cell);
ii) transmission of viruses from person to person involves several routes including skin contact, coughing, sneezing, insect bites/animal bites and through bodily fluids like blood and bodily secretions like saliva including sexual contacts.
Viruses and other types of infectious agents that are transmitted from insect or human bites are also called as zoonotic viruses . Upon infection, a virus first gets access into the cell followed by hijacking the intracellular machinery of that cell, i.e. utilizing it for producing more viruses. Importantly viruses have a genetic material either deoxyribonucleic acid or ribonucleic acid and protein covering called capsid .
As such their entry into the cell is mainly delivery of their genetic material, and this further utilizes human cell resources. Importantly, there are two main group of viruses, one that integrates into the genetic material of human cell and becomes a part of the genome; an example is the human immunodeficiency virus (HIV) and the others non-integrating viruses.
Viral Infections Impact on the Human Body
Subsequent to an infection, a virus can either kill the cell and release an enormous progeny of viruses into the body fluid mainly blood that spreads it all across the body. A decline in the number of CD4 positive cells associated with human immunodeficiency virus (HIV) infection is considered as a biomarker for the viral infection and its severity .
Alternatively, some type of viruses just keep utilizing the human cellular machinery and stay dormant. Certain viruses even get reactivated after dormancy and cause infection. As such damage to cell inflicted by the virus is ultimately translated into tissues deformity followed by dysfunctional organs and the system. Viral hepatitis is an example, where the viral infection damages the liver and person either has to get a liver transplant or face the fatality resulting from failing liver .
Several viruses affect the circulatory system and deplete cellular elements circulating within the body. A few to worth mention are HIV, human herpes viruses (HHV), dengue fever, EBOLA, and Hantaviruses.
2. Bacterial Infections
Bacteria are single cell living organisms that can be seen only under the microscope. A variety of bacteria exist in the body and surrounding environments of the human being, however, a few causes diseases .
Morphologically, bacteria are classified into three major groups, helical (spirilla), rod-shaped (bacilli) and spherical (cocci). Bacteria causes both superficial (skin) and systemic infections in the human body. Among the skin diseases, few important are cellulitis, folliculitis, and boils, whereas systemic infections fall into three categories including:
a. Foodborne – gastrointestinal infections caused by Vibrio cholerae, and Salmonella typhi causing typhoid fever
b. Sexually transmitted – Several bacterial infections are sexually transmitted including Neisseria gonorrhoeae causing gonorrhea and Chlamydia trachomatis infections leading to inflammatory diseases.
c. Miscellaneous bacterial infections – There are several other bacterial infections transmitted from human to human through the air, bodily fluids, open wounds and several other ways. Few to mention are bacterial respiratory tract infection and meningitis causing bacterial species.
Bacterial infection causing skin diseases and the other within the body are treated with antibiotics. An important issue related to the treatment of bacterial infections is the emergence of resistance bacteria where antibiotics fail to provide desired response.
Bacterial Infections Impact on the Human Body
Infectious bacteria harm human body in a number of ways. Among the several four most important are:
i) Infectious bacteria utilize the necessary nutritional elements required for various activities in the body;
ii) Bacterial infection can either directly damage the human cells or produce certain toxins that will damage human tissues by inducing toxicities;
iii) Certain bacterial infections are associated with hypersensitivities leading to allergic reaction ultimately disturbing overall bodily homeostasis .
Our body protects us from bacterial infections in a variety of ways like natural defense provided by the skin, bodily secretions, tears, saliva having bacteria killing enzymes, acidic environment of the stomach. Within the gastrointestinal tract and mainly in the colon all humans have a vast population of beneficial bacteria known as normal flora that helps in the digestion of food. As such our body has mechanisms to promote the growth of beneficial bacterial species and controlling the one harming our body.
In case of infections causing bacteria cross the natural barriers of the human body (innate defense), the immune system gets activated. This system is comprised of innate and acquired setup. For example, cells known as phagocytes (macrophages) circulating in the blood and present in tissues, swallow the bacteria and kill them by tearing into pieces.
Besides this killing, the human body produces a few types of antibodies against the infectious bacteria that circulate in the blood, identify the infectious bacteria followed by its killing. This protective system of human body controlling various infections including, viruses, bacteria, fungi, and parasites is highly integrated comprising natural killer cells, production of antibodies and the complement system .
3. Fungal Infections
Fungal infections are caused by a group of organisms called fungi. When compared with bacteria, fungi have well-defined nucleus having chromosomes thus falling into the eukaryotic group compared with bacteria (prokaryotes). Reproduction in fungi is mainly through spores, and like humans, they are heterotrophic (dependent on external nutrients).
Among thousands of fungal species existing on this earth only a few cause diseases in humans. The fungal spores are mainly involved in the transmission of fungal infections from person to person . Inhalation of spores and their access to lungs or superficial attachment to the skin initiate the fungal disease.
Fungal Infections and Human Body
Both superficial and systemic fungal infections have been reported among people. Superficial fungal infections mainly infect both the soft tissues of skin and hard tissue like nails and hair. Identification of superficial fungal infections is associated with responses like itching, inflammation, and redness. Typical superficial fungal infection reported in humans are athlete’s foot, superficial yeast infections.
There are a variety of systemic fungal infection. However, the majority are the opportunistic infection, becoming prominent when the body defense system is weakened particularly in diseases like acquired immunodeficiency syndrome (AIDS) and certain types of cancers. The majority of pathogenic fungi exist in two forms a small circular yeast and extremely branched structure known as hyphal form.
Mainly the opportunistic fungi, within the human body are converted to hyphal form and a bunch of hyphae besides damaging host cells also impact the circulatory systems and proper supply of tissues nutritional requirements. Important fungal infection among humans are aspergillosis, candidiasis, and mucormycosis .
4. Parasitic Infections
Parasites are the organisms depending on their host for their nutritional requirements and lead to several infections among humans . Parasitic infections among people are due to three major group of parasites including protozoa, ectoparasite, and helminths.
Examples are Entamoeba causing amoebic dysentery belongs to protozoa, flukes causing human gastrointestinal and liver disorders belongs to helminths and ectoparasites involved in the transmission of several blood-borne diseases. Parasitic infections are mainly prevalent in tropical and subtropical regions and sometimes described as neglected tropical diseases (NTD) .
Human Body and Parasites
Transmission of parasites from human to human or from non-human species to human involve mainly two routes, i.e. fecal-oral route (contaminated drinking water and person to person contacts) or transmission through arthropods (for parasites in human blood). Among all, malaria causing parasite mainly the Plasmodium falciparum has received more attention due to its extensive prevalence. This parasite damages the human erythrocytes and is vector born in nature transmitted person to person through mosquitoes .
There are a number of ways through which parasites destroy the human body and tissues. For example, blood in stool is associated with injuries linked to intestinal parasites like. A parasite Dictyocaulus filaria obstructs airways of lungs thus causing pneumonia and collapsing of lungs. Certain parasites like Fasciola hepatica causes extensive bleeding through releasing anti-clotting agents in wounds.
5. Toxic Substances and Human Health
In the human health perspective, a toxin is a substance having damaging effects on people's health. There are a variety of toxins either produced by infectious agents, living organisms or produced synthetically. Sometimes toxins are added into the environment through various human activities. For example, lead poisoning is a major issue linked to gasoline and industrial emissions.
Even in the old days lead was used in house paints that is no longer practiced in the aftermath of its recognition as a toxic substance. Similarly, areas having higher arsenic levels (a toxin) are prone to ingest this toxic chemical in the drinking water. A most common toxin, nicotine, and associated other oncogenic toxic materials are delivered into the human body through smoking and passive transmission to the people around smokers.
Besides environmental toxins impacting people's health, several types of microorganisms damage human tissues by secreting certain toxins, an example is cholera toxin causing gastrointestinal disorders in human  and botulinum toxins having several complications.
Toxins in Humans
Toxin emerging from the industrial emission have been a significant risk hazard in the industrialized nations. The industrial fumes and effluents are rich in toxic material that can get into human either through inhalation or absorption into food material thus getting access to the human body. The human body has a specific way of decreasing the toxicity of harmful material through detoxification processes . Several body organs including skin, lymph, lungs, kidney, and liver are involved in the detoxification processes.
The liver is a major organ of the human body entrusted with detoxification processes. Major processes are converting toxic substances to highly soluble forms so they can be readily excreted in the urine. Several other detoxification mechanisms are converting highly toxic substances into less toxic through chemical modification within the human body.
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