Health is described as the state of complete physical, mental, and social well being. Being healthy is far more than just being free from diseases.

The disease is a condition of disturbed functioning of the body caused by infection, defective diet, heredity, environment, or deprived condition of the brain. Health is a state of complete physical, mental, and social well being.

The disease may be a response to

  • Environmental factors (as malnutrition, industrial hazards, or climate)
  • Specific infective agents (as worms, protozoans, fungi, etc)
  • Inherent defects of the organism (as genetic anomalies.)
  • Combination of these factors

Causes of Diseases/Disease Agents

A disease agent is an organism, substance, or force which causes disease due to its excessive presence, deficiency, or absence.

  • Pathogens/Biological Agents: They are biological entities which cause infectious diseases, e.g., viruses (mumps, chickenpox, smallpox), mycoplasma (e.g, bronchitis, acute leukemia), chlamydia (e.g, trachoma), bacteria (e.g. cholera, tetanus), fungi (ringworm, thrush, moniliasis, pulmonary aspergillosis), protozoa (e.g. giardiasis, sleeping sickness), helminths (e.g., filariasis, ascariasis, taeniasis), other organisms (e.g., scabies).
  • Nutrient Agents: Deficiency of vitamins (e.g., beriberi, scurvy, night blindness), minerals (e.g., anaemmia, rickets), carbohydrates, fat, and proteins (e.g., kwashiorkar, marasmus), or excess of food (e.g., obesity).
  • Chemical Agents: Endogenous Agents– Excess presence of uric acid, reduced secretion of ADH (diabetes insipidus) or insulin (diabetes mellitus). Exogenous Agents- Pollutants (e.g., pneumoconiosis), allergens (allergy).
  • Physical Agents: Heat (e.g., stroke), cold (frostbite), radiations, sound (impaired hearing), humidity, etc.
  • Mechanical Agents: Fractures, sprains, dislocation, injury, chronic friction.
  • Genetic Agents: Excess or deficiency of chromosomes, mutations, harmful alleles, e.g, colour blindness, albinism, haemophilia, Turner’s syndrome.
DiseaseCausative Agent
PlaguePasteurella pestis
CholeraVibrio comma (Vibrio cholera)
TetanusClostridium tetani
AnthraxBacillus anthracis
Whooping coughBordetella pertussis
Human papillomavirus infectionHuman papillomavirus
Acquired Immune Deficiency Syndrome (AIDS)Human Immunodeficiency Virus (HIV)
HepatitisHepatitis A, Hepatitis B, Hepatitis C, Hepatitis D, Hepatitis E viruses
ChickenpoxVaricella zoster virus (VZV)
MeningoencephalitisNaegleria fowleri (amoeba)

Classification of Disease

According to a very broad classification, diseases can also be classified under the following – physical diseases, mental diseases, infectious diseases, non- infectious diseases, deficiency diseases, inherited diseases, degenerative diseases, social diseases, self-inflicted diseases.

Depending on certain characteristics, diseases can also be classified as an acute disease where the onset of the disease is sudden, lasts for a short time, with rapid changes; and chronic diseases where the effects of the disease can last for months or years.

Diseases can also be classified in other ways, such as communicable versus non-communicable diseases.

TypeExplanationExample
Anatomic ClassificationThis type refers to the affected organ or tissueHeart disease
Topographical ClassificationFurther classified into types such as vascular disease, chest disease, gastrointestinal disease, and abdominal diseases. These are then handled by specializations in medicine that follow these topographical classificationsAn ENT specialist (Ear-Nose-Throat)
A Gastroenterology specialist etc.
Physiological ClassificationThis type includes diseases that affect a process or a function (such as metabolism, digestion or respiration)Diabetes
Pathological ClassificationThis type considers the nature of the disease. For instance, cancer is associated with uncontrolled cell growth, and there are variations or types in the disease.Neoplastic diseases (uncontrolled cell growth that are characteristic of cancer)
Inflammatory diseases (autoimmunity)
Epidemiological ClassificationThis classification refers to the rate of occurrence, distribution and the control of the disease in a population.Epidemic diseases such as plague and Influenza pandemic of 1918–1919

Infectious Diseases

  • Diseases that spread from one person to another are called communicable diseases.
  • They are usually caused by microorganisms called pathogens (fungi, rickettsia, bacteria, viruses, protozoans, worms).
  • When an infected person discharges bodily fluids, pathogens may exit the host and infected a new person (sneezing, coughing etc).
  • Examples include Cholera, chickenpox, malaria etc.

Non-infectious Diseases

  • These diseases are caused by pathogens, but other factors such as age, nutritional deficiency, gender of an individual, and lifestyle also influence the disease.
  • Examples include hypertension, diabetes, and cancer.
  • They do not spread to others and they restrain within a person who has contracted them.
  • Alzheimers, asthma, cataract and heart diseases are other non-infectious diseases.

Bacterial Diseases

Bacteria are prokaryotes, a minuscule single-celled organism that grows well in varied environments. They can live inside soil, in the ocean, and inside the human bowel. They can be differentiated by, by their shape, the nature of their cell walls and genetic differences.

Bacterial diseases include any type of illness caused by bacteria. Bacteria are a type of microorganism, which are tiny forms of life that can only be seen with a microscope. Other types of microorganisms include viruses, some fungi, and some parasites.

Examples are

  • Diphtheria
  • Anthrax
  • Pneumonia
  • Leprosy
  • Tuberculosis
  • Plague– Plague is caused by the bacteria Yersinia pestis, a zoonotic bacterium usually found in small mammals and their fleas.
  • Meningitis– Meningitis — an infection of the tissues that cover and protect the brain and spinal cord — can be caused by viruses, bacteria, or fungi.
  • Cholera
  • Tetanus
  • Typhoid fever
  • Lyme disease
  • Whooping cough
  • Gonorrhea – It is a sexually transmitted disease. It is caused by the gonococcus, Neisseria gonorrhoeae—a bacterium.
  • Syphilis – a systemic disease that is caused by the spirochete bacterium, Treponema pallidum. Syphilis is usually a sexually transmitted disease, but it is occasionally acquired by direct nonsexual contact with an infected person, and it can also be acquired by an unborn fetus through infection in the mother.

Viral Diseases

Virus – A virus is a small infectious agent that replicates only inside the living cells of other organisms. They’re made up of a piece of genetic material, such as DNA or RNA, that’s enclosed in a coat of protein.

A viral disease is any illness or health condition caused by a virus.

Viruses invade cells in your body and use components of those cells to help them multiply. This process often damages or destroys infected cells.

Examples are

  • Influenza-Flu by influenza virus
  • Common Cold- Rhinovirus
  • Hepatitis A– Liver
  • Norovirus-Gastro-intestinal illness
  • Rotavirus– Diarrhea
  • Human Papilloma Virus (HPV)
  • Hepatitis B– Inflammation in the liver
  • HIV
  • Measles
  • Rabies
  • Smallpox
  • Polio
  • Rubella
  • Chickenpox
  • Japanese Encephalitis
  • Zika Viral Fever
  • Yellow Fever
  • MERS-CoV (Middle East respiratory syndrome)
  • SARS – Severe Acute Respiratory Syndrome

Protozoan Diseases

Protozoan infections are parasitic diseases caused by organisms formerly classified in the Kingdom Protozoa.

Protozoal disease, a disease caused by protozoans. These organisms may remain in the human host for their entire life cycle, but many carry out part of their reproductive cycle in insects or other hosts. For example, mosquitoes are vectors of plasmodium, the cause of malaria.

They are usually contracted by either an insect vector or by contact with an infected substance or surface and include organisms that are now classified in the supergroups ExcavataAmoebozoaSAR, and Archaeplastida.

Examples –

  • Malaria
  • Amoebiasis
  • Trichomoniasis
  • African Sleeping sickness or Trypanosomiasis
  • Leishmaniasis or Kala-azar
  • Giardiasis
  • Balantidiasis
  • Toxoplasmosis
Name of the DiseaseVectorPathogenesis
MalariaFemale Anopheles mosquitoThe parasite attacks the liver and RBCs. It multiplies within liver cells, enters the bloodstream and ruptures RBCs. It releases a toxic substance called ‘hemozoin’, which causes fever. The sporozoite is the infectious stage
Amoebiasis or Amoebic dysenteryNone.
It gets transmitted by contaminated food or water
Invades intestinal mucosa and spreads to other parts like liver. Causes dysentery and liver abscesses.The infected stage is trophozoites
African Sleeping sickness or TrypanosomiasisTsetse flyB-lymphocyte proliferation leading to tissue damage
TrichomoniasisSexually transmitted disease (STD)Destroys epithelial cells and cytotoxic substances are released. Vaginal pH increases and the number of leukocytes also increases in response to the toxic substance released by the pathogen
ToxoplasmosisTransmission by contaminated water and soil or get attached to fur of animalsSporozoites penetrate the intestinal cells and multiply in the intestine. It invades the lymphatic system and blood and damages the tissue leading to necrosis
BalantidiasisPigsExcystation occurs in the small intestine. Sporozoites migrate to the colon
GiardiasisNone.
It gets transmitted by contaminated food or water
Mucosal damage is related to the mucosal inflammation and release of lectin or proteinases. Malabsorption may also be due to inhibition of pancreatic enzymes and depletion of bile concentration
Leishmaniasis or Kala-azarFemale Sandflies (of the genus Phlebotomus)The flagellated promastigotes of the parasite bind to macrophages present in the skin. There is marked suppression of cell-mediated immunity

Zoonotic Diseases

The word ‘Zoonosis’ (Pleural: Zoonoses) was introduced by Rudolf Virchow in 1880 to include collectively the diseases shared in nature by man and animals.

Later WHO in 1959 defined that Zoonoses are those diseases and infections which are naturally transmitted between vertebrate animals and man.

Zoonotic Diseases are diseases that can be passed between animals and humans. Viruses, bacteria, parasites, and fungi can cause zoonotic diseases.

Examples –

  • Chikungunya
  • Malaria
  • Yellow Fever
  • Zika Virus Disease
  • Dengue Fever
  • Ebola
  • Hepatitis E
  • Rabies
  • Japanese Encephalitis
  • Filariasis
  • Lyme disease
  • Babesiosis
  • Ehrlichiosis
  • Ringworm
  • Swine flu, also known as the H1N1 virus – caused by type A influenza virus.
  • West Nile virus – causes a viral infection that is typically spread by mosquitoes and can cause neurological disease as well as death.

Neglected tropical diseases

Neglected tropical diseases (NTDs) are a diverse group of tropical infections that are especially common in low-income populations in developing regions of Africa, Asia, and the Americas.

Populations living in poverty, without adequate sanitation and in close contact with infectious vectors and domestic animals and livestock are the worst affected.

The World Health Organization (WHO) is a specialized agency of the United Nations that looks into matters regarding public health.

They are caused by a variety of pathogens such as viruses, bacteria, protozoa and helminths.

Examples –

  • Buruli Ulcer
  • Rabies
  • Dengue
  • Chikungunya
  • Leprosy (Hansen’s disease)
  • Lymphatic Filariasis
  • Yaws
  • Trachoma
  • Schistosomiasis

West Nile Virus

  • West Nile Virus (WNV) can cause neurological disease and death in people. WNV is commonly found in Africa, Europe, the Middle East, North America, and West Asia.
  • WNV is maintained in nature in a cycle involving transmission between birds and mosquitoes. Humans, horses, and other mammals can be infected.
  • West Nile Virus (WNV) is a member of the flavivirus genus and belongs to the Japanese encephalitis antigenic complex of the family Flaviviridae. Birds are the natural hosts of the West Nile virus.

Leishmaniasis

  • Historically known as “Aleppo boil,” this parasitic infection has recently, as the name suggests, become a problem among Syrian refugees.
  • Producing disfiguring skin ulcerations, and occasionally spreading to internal organs with fatal consequences, the increase of cases turning up in Europe among migrants has made it the subject of considerable media interest.
  • Leishmaniasis is spread by the bite of the sandfly, however, which means it has a northern limit to its range.

Tuberculosis (TB)

  • TB is caused by a bacterium called Mycobacterium tuberculosis, belonging to the Mycobacteriaceae family consisting of about 200 members.
  • In humans, TB most commonly affects the lungs (pulmonary TB), but it can also affect other organs (extra-pulmonary TB).
  • TB is a very ancient disease and has been documented to have existed in Egypt as early as 3000 BC.
  • TB is a treatable and curable disease.
  • TB is spread from person to person through the air. When people with lung TB cough, sneeze or spit, they propel the TB germs into the air.
  • Symptoms: Common symptoms of active lung TB are cough with sputum and blood at times, chest pains, weakness, weight loss, fever and night sweats.
  • NIKSHAY– A web-based solution for monitoring of TB patients to monitor the Revised National Tuberculosis Programme (RNTCP) effectively, a web-enabled and case-based monitoring application called NIKSHAY has been developed by National Informatics Centre (NIC).
  • Global Impact of TB:
    • In 2019, 87% of new TB cases occurred in the 30 high TB burden countries.
    • Eight countries accounted for two thirds of the new TB cases:
      • India, Indonesia, China, Philippines, Pakistan, Nigeria, Bangladesh and South Africa.
      • India reported 1.8 million TB cases between January and December 2020 as compared to 2.4 million the year before.
    • In 2019, MDR-TB remained a public health crisis and a health security threat.
      • MultiDrug Resistant Tuberculosis (MDR-TB) is a strain of TB that cannot be treated with the two most powerful first-line treatment anti-TB drugs. Extensively Drug Resistant Tuberculosis (XDR-TB) is a form of TB caused by bacteria that are resistant to several of the most effective anti-TB drugs.
  • BCG Vaccine:
    • BCG was developed by two Frenchmen, Albert Calmette and Camille Guerin, by modifying a strain of Mycobacterium bovis (that causes TB in cattle). It was first used in humans in 1921.
    • In India, BCG was first introduced in a limited scale in 1948 and became a part of the National TB Control Programme in 1962.
    • In addition to its primary use as a vaccine against TB, it protects against respiratory and bacterial infections of the newborns, and other mycobacterial diseases like Leprosy and Buruli’s ulcer.
    • It is also used as an immunotherapy agent in cancer of the urinary bladder and malignant melanoma.
    • One intriguing fact about BCG is that it works well in some geographic locations and not so well in others. Generally, the farther a country is from the equator, the higher is the efficiency.
      • It has a high efficacy in the UK, Norway, Sweden and Denmark; and little or no efficacy in countries on or near the equator like India, Kenya and Malawi, where the burden of TB is higher.

Leprosy

  • Leprosy, also known as Hansen’s disease, is a chronic infectious disease caused by Mycobacterium leprae.
  • The disease mainly affects the skin, the peripheral nerves, mucosal surfaces of the upper respiratory tract, and the eyes.
  • Leprosy is known to occur at all ages ranging from early infancy to very old age. Leprosy is curable and early treatment averts most disabilities.
  • Leprosy is curable with a combination of drugs known as multidrug therapy (MDT), as the treatment of leprosy with only one antileprosy drug (monotherapy) will result in the development of drug resistance to that drug.
  • The combination of drugs used in the MDT depends on the classification of the disease. Rifampicin, the most important antileprosy medicine, is included in the treatment of both types of leprosy.
  • For the treatment of patients with multibacillary leprosy, WHO recommends a combination of rifampicin, clofazimine, and dapsone; for patients with paucibacillary leprosy, MDT uses a combination of rifampicin and dapsone.

Alzheimer’s Disease

  • It is a progressive brain disorder that typically affects people older than 65. It destroys brain cells and nerves and disrupts the message-carrying neurotransmitters.
  • When it affects younger individuals, it is considered early onset.
  • Eventually, a person with Alzheimer’s loses the ability to perform day-to-day activities.
  • Symptoms include memory loss, difficulty in completing familiar tasks, confusion with time or place, problems in speaking and writing, decreased or poor judgment, and changes in mood and personality.
  • There is no cure for Alzheimer’s because its exact causes are not known. Most drugs being developed try to slow down or stop the progression of the disease.

Parkinson’s Disease

  • Parkinson’s disease is a chronic, degenerative neurological disorder that affects the central nervous system.
  • It damages nerve cells in the brain dropping the levels of dopamine. Dopamine is a chemical that sends behavioral signals from the brain to the body.
  • The disease causes a variety of “motor” symptoms (symptoms related to the movement of the muscles), including rigidity, delayed movement, poor balance, and tremors.
  • Medication can help control the symptoms of the disease but it can’t be cured.
  • It affects the age group from 6 to 60 years. Worldwide, about 10 million people have been affected by this disease.

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