Diseases and its Causes: Definition, Classification of Diseases and Causes

At all times, our body organs perform a variety of specialised functions. These interrelated functions of several body organs help to sustain good health. A sickness or condition is caused by anything that hinders cells, tissues, or body organs from functioning properly. But why do we get sick in the first place? What are those Diseases and its Causes? A disease is any disorder or malfunction of the body or mind that impacts one’s health. In disease, the body’s health status is said to be compromised.

The origins of disease were based entirely on mythology, superstition, and malevolent spirits in the early days. Every disease has a specific cause, according to science. Science and technology have succeeded in understanding the symptoms, prevention, and cure of several diseases, in addition to their causes. A disease can be caused by a number of factors. Every disease has distinct symptoms that allow us to distinguish between diseases.

What is Disease?

The term disease originated from the old French word desaise, which means lack of ease. A disease is a condition that deteriorates the normal functioning of a cell, tissues, organ or organism and has a specific cause and clinical symptoms. Pathology or etiology is the branch of medical science that deals with the study of diseases, their effects, and causes of diseases.

Disease: Related Terminology

1. Zoonotic Disease: The term refers to the disease that is transmissible from animals to humans and is called zoonotic disease.
2. Acute: A condition that begins suddenly and is sometimes severe, but for a short duration.
3. Chronic: Signifies a recurring, persistent abnormal condition with mild symptoms, like heart disease, diabetes.
4. Contagious: Diseases likely to spread through various means.
5. Non-contagious: Diseases that do not spread by direct or indirect contact.
6. Immunity: The ability of an organism to check a particular infection by the action of specific antibodies or sensitised leucocytes.
7. Innate immunity: Innate Immunity is the immunity that exists from birth as a result of a person’s genetic constitution or physiology.
8. Acquired immunity: It is adaptive immunity, either obtained from the development of antibodies in response to exposure to an antigen (pathogens) or from vaccination.
9. Vaccine: A biological preparation obtained from the weakened causative agent of a disease that provides immunity by stimulating the production of antibodies.
10. Vaccination: Injection of a mild dosage of a microbe to stimulate the immune system against the microbe, thereby preventing disease.
11. DTP: A vaccine for diphtheria, tetanus, and pertussis.
12. Malignant: Indicates the presence of cancerous cells.
13. Prognosis: The predicted outcome of disease and treatment.
14. Diagnosis: Identification of disease by the examination of symptoms.

Classification of Diseases

Diseases can be categorised into different types based on the following criteria:

1. The extent of occurrence: The diseases restricted to a specific population or geographical region are called endemic diseases. The diseases that spread over multiple countries are called epidemic diseases. Those distributed worldwide are called pandemic diseases. The diseases that occur infrequently and irregularly in a community are called sporadic diseases.
2. Communicability: The diseases that cannot be transmitted from one person to another are called non-communicable diseases or non-infectious diseases. In contrast, communicable diseases can be transmitted from one person to another through various means.
   Communicable diseases are mainly caused by pathogens, including bacteria, viruses, protozoans, fungi, flatworms, and roundworms. At the same time, lack of a nutrient-rich diet, metabolic and genetic disorders, allergies, and cancer are the causes of non-communicable diseases.
3. Kinds of transmitting agent: The disease-causing microbes can be transmitted through various physical and biological agents such as air, water, insects (vectors), blood, etc. Communicable diseases are transmitted through multiple agents.
4. Duration of diseases: The diseases that are short-term with intense symptoms are called acute diseases. The diseases that are long-term, with symptoms lasting for months and years, are called chronic diseases.

Causes of Non-communicable Diseases

These are non-infectious or non-contagious diseases that are caused due to several intrinsic and extrinsic factors. These diseases can be classified as follows based on their causes.

Nutritional Deficiency Diseases

Nutrients are the external factors that are consumed by an organism to obtain energy. The absence of a nutrient-rich diet makes a person unhealthy and more susceptible to diseases. Deficiency of nutrients in the diet results in the number of following deficiency diseases in human beings:

Diseases Caused Due to Environmental Pollutants

Environmental pollutants are the external factors that negatively affect the health of a person and cause several diseases. Air and water pollutants cause different diseases such as chronic obstructive pulmonary disease, lung cancer, cardiac diseases, kidney diseases, Minamata disease, etc.

Metabolic Disorders

Metabolic diseases result from failure of metabolism that causes the body to have excess or small amounts of the essential substances needed to stay healthy. Some inherited metabolic disorders are as follows:
1. Wilson’s disease:  It is caused due to the accumulation of copper in the liver, brain and other vital organs.
2. Gaucher’s disease: It is the result of a buildup of certain fatty substances in certain organs, particularly the spleen and liver.
3. Hunter’s syndrome: It is caused due to the lack or malfunctioning of an enzyme iduronate \(2\)-sulfatase that is responsible for breaking down certain complex molecules. The buildup of massive amounts of complex molecules eventually causes permanent and progressive damage to mental development, organ function and physical abilities.
4. Porphyria: It refers to a group of disorders that are caused due to the buildup of natural chemicals that produce porphyrin in the body.

Genetic Disorders

Genetic disorders are due to the mutation in a gene. About 4,000 diseases of humans have recognised genetic basis. The mutant allele may be recessive or dominant and may be found on autosomal or sex chromosomes. These diseases affect 1-2% of the human population. These can be categorised into two types:
Mendelian Disorders: Some examples of Mendelian disorders are as follows:
1. Sickle cell anaemia: It is a genetic disease in which afflicted individuals contain red blood cells under low oxygen tension and become elongated and curved.It is caused due to the mutation in the haemoglobin-beta gene located on chromosome 11 that is responsible for synthesising the proteins needed for haemoglobin in red blood cells.
2. Phenylketonuria: It is an inherited error of metabolism caused by the deficiency of phenylalanine hydroxylase that leads to mental retardation. The changes can be traced to a tiny mutation in a single gene on chromosome 12. It is an inherited error of metabolism caused by the deficiency of phenylalanine hydroxylase.
3. Cystic fibrosis:  It is the inability to transport salt in certain kinds of cells in the lungs and in the pancreas. All characteristics of cystic fibrosis arise from a mutation in a gene located on chromosome 7.
4. Huntington’s disease: It is a brain disorder due to a dominant gene on autosomal chromosome 4.
5. Haemophilia A & B: These are hereditary blood disorders. Haemophilia A and B are caused due to the absence of blood clotting factor VIII (antihemophilic factor) and factor IX (Christmas factor), respectively.

Chromosomal disorders: These include the following syndromes:
1. Down’s syndrome: It is caused by the presence of an extra chromosome number 21 (three 21 chromosomes). This chromosomal abnormality adversely affects the physical and intellectual development of the individual.
2. Klinefelter’s syndrome: It is an abnormal condition of sexual characteristics with an extra X-chromosome. These individuals have the karyotype (2A+XXY) and (2A+XXX) respectively in males and females.
3. Turner’s syndrome: It is an abnormality of sex chromosomes in females with a single X chromosome (2A+XO) 45 chromosomes.

Allergies

1. Excessive and inappropriate responses to allergens characterise allergic diseases.
2. Common allergens include dust, pollen, animal dander, mites, feathers, latex, and certain food products like nuts and gluten.
3. Allergy is due to the release of chemicals like histamine and serotonin from the mast cells.
4. Antibodies to allergens combine with receptors on the mast cells forming a complex that causes the cells to burst and release histamines that are responsible for inflammatory reactions.
5. Some common allergic diseases include asthma (which affects the bronchi and bronchioles of the lungs), Hay fever (causing cold & cough like symptoms).
6. Anaphylactic shock is a severe degree of anaphylaxis (an extreme allergic reaction) that may result in a sudden attack of wheezing, falling blood pressure and, therefore may be fatal.

Cancer

Cancer is an abnormal and uncontrolled division of cells. Causes of cancer include certain physical factors (ionising radiation, pollution),  chemical factors (tobacco smoking, tobacco chewing), biological factors (mutated alleles of genes called oncogenes) and oncoviruses.  Breast cancer, prostate cancer, and leukaemia are different types of cancer.

Hormonal Disorders

Hormones are chemical messengers that are produced by endocrine glands. The excess and less secretion of hormones cause several diseases that can be listed as follows:

Causes of Communicable Diseases

Communicable diseases are infectious diseases and are transmitted from one person to another. Communicable diseases are mainly caused by various organisms, like bacteria, viruses, fungi, protozoans, helminths, etc.

Bacteria

Bacteria are unicellular prokaryotes. Not all strains of bacteria are harmful, but some of them cause diseases. A bacterial infection is the propagation of a deadly strain of bacteria on or inside the body. Bacterial diseases occur when pathogenic bacteria get into the body and begin to reproduce or grow in normally sterile tissues (from bacteria).

Human diseases caused by bacteria

Fig: An Overview of Bacterial Infections

Viruses

Viruses are infectious microscopic agents that replicate inside the body of living organisms and derive nutrition at the expense of their host. Viruses infect all life forms, including plants, animals, and microbes. Virus term is derived from a Latin word meaning venomous substance.

Human diseases caused by viruses

Fig: An Overview of Viral Infections

Protozoans

Protozoans are unicellular, eukaryotic organisms that are either-living or parasitic. Parasitic protozoans cause diseases in plants and animals, including humans. These can move by cilia, flagella, and pseudopodia.

Human diseases caused by protozoans:

Helminths

Helminths are parasitic worms that obtain their nutrition from the host and cause diseases to the host. Helminths include Platyhelminthes (flatworms) and nematodes (roundworms).

Animal & Human diseases caused by helminths

Fungi

Fungi can be unicellular or multicellular eukaryotic organisms. They feed upon the dead and decaying organic matter. Not all fungi are harmful. Some are used in industry, such as yeast for fermentation, some are edible such as mushrooms.

Human diseases caused by fungi

Summary

The term disease originated from the old French word desaise, which means lack of ease. It refers to without ease or discomfort. Diseases can be abnormal conditions that adversely affect health. The health of living organisms is negatively affected due to physiological abnormalities, psychological reasons, or pathogens.

Diseases may be a response to specific infective agents such as bacteria, viruses, protozoans, fungi, helminths, etc. Beyond these, it can be the result of hazardous environmental factors that mainly include pollutants. Other aspects of diseases include genetic and metabolic abnormalities and an imbalanced diet.

Frequently Asked Questions

We have provided some frequently asked questions on Disease and its Causes here:

Q.1. What is a disease?
Ans: A disease is a condition that deteriorates the normal functioning of a cell, tissues, organ or organism and has a specific cause and clinical symptoms.

Q.2. What are the causes of diseases?
Ans: Diseases can be caused by pathogens, including bacteria, viruses, protozoans, fungi, and worms. Besides this, diseases can occur due to nutritional deficiency, metabolic disorders, and hormonal imbalance.

Q.3. What is a curable disease?
Ans: The diseases that can be treated and recovered completely with no longer clinical symptoms are called curable diseases.

Q.4. What are the ten most common diseases?
Ans: Cancer, diabetes, influenza, common cold, pneumonia, asthma, malaria, bronchitis, amoebic dysentery, and hypertension are the most common diseases.

Q.5. What are the different types of diseases?
Ans: Diseases can be classified into the following types: communicable diseases, non-communicable diseases, hereditary diseases, and physiological diseases.

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