Diseases and their classification

Disease: Cause and Symptoms

The disease is a state of discomfort caused by various factors like infections, and deficiency of vitamins.  The disease is defined in various ways. The following are a few definitions of disease.

Cause of disease:

Many factors, like nutritional deficiency of vitamins and protein, can cause disease. Infectious organisms like bacteria, viruses, Protozoans, fungi, helminths, etc., can also cause disease. Further, genetic factors also cause diseases.

Types of disease:

The disease can be classified in various ways. Based on whether it is present at birth or developed after birth, it can be classified as called congenital or acquired disease, respectively. The disease can be genetic or non-genetic. Most but not all genetic diseases are congenital. Huntington’s chorea is a genetic disorder characterized by involuntary, jerky movements manifest at a later age; hence it is not congenital. In contrast, microcephaly (smaller size head) is a congenital disease caused by the Zika virus infection during pregnancy, passing from the mother to the foetus.

A disease can also be classified as chronic or acute based on its severity. A chronic disease lasts for a long time and causes severe damage, while an acute disease lasts for a short period and does not cause much harm.

A disease can be classified as infectious or non-infectious based on whether it spreads among people or remain confined to the affected person only.

If some living organism causes a disease, it can be called an infectious disease. It can be further classified based on the cause of infection, whether a bacterium, virus, helminth, etc.

The following flow chart gives a classification of diseases and a few examples.  

Diseases and their classification
Diseases and their classification

Disease caused by the deficiency of vitamins

  • Vitamins are small, non-nutritious biomolecules required in minimal quantity. They are sometimes a precursor to coenzymes.
  • Casimir Funk coined the term Vitamin in 1912.

The Vitamins, their function, and the deficiency symptoms are given in the following table.



Deficiency disease/symptom

Fat Soluble Vitamins

Vitamin A (retinol)

Roles in vision, growth, reproduction


Vitamin D *

Regulation of calcium and phosphate metabolism

Rickets (children): skeletal deformities, impaired growth Osteomalacia (adults): soft, bending bones

Vitamin E



Inhibition of sperm production; lesions in muscles and nerves (rare)

Vitamin K #

Blood coagulation

Subdermal hemorrhage

Water Soluble Vitamins

Thiamine (B1)


Beriberi (weight loss, heart problems, neurological dysfunction)

Riboflavin (B2)


Cheilosis and angular stomatitis (lesions of the mouth), dermatitis

Pyridoxine (B6)


Depression, confusion, convulsions

Nicotinic acid (niacin, B3)


Nicotinic acid (niacin)

Nicotinic acid (niacin)





Rash about the eyebrows, muscle pain, fatigue (rare)

Folic acid


Anemia, neural-tube defects in development

Cyanocobalamin (B12)


Anemia, pernicious anemia, methylmalonic acidosis

C (ascorbic acid)


Scurvy (swollen and bleeding gums, subdermal hemorrhages)

* Vitamin D is not a vitamin as it is not required in the diet. It is a UV-photolysis product of &-dehydrocholesterol (an intermediate of cholesterol biosynthesis). The most abundant form of vitamin D is D3 (cholecalciferol).

# In plants, Vitamin K is present as phylloquinone (vitamin K1) and as menaquinone (vitamin K2) in animals.

Infectious Diseases:

 A few (infectious) diseases, their cause, and their symptoms are given in the following table.




Symptoms/ mode of entry/ pathological test


Salmonella typhi

(a bacterium)

  • High fever (39° to 40°C), weakness, stomach pain, constipation, headache, and loss of appetite;
  • Spread through contaminated food and water;
  • Widal test is used to confirm Salmonella infection.


Streptococcus pneumoniae (Pneumococcus),

Mycoplasma pneumoniae Haemophilus influenzae



  • Lung alveoli get filled with fluid causing respiratory problems, fever, chills, cough, and headache; in severe cases, the lips and fingernails may turn gray to bluish; parapneumonic effusion, empyema, septic shock;
  • It spreads through sneezing and contaminated stuff like tissue paper, etc. 
  • Spread through air



Amoebic Dysentery


Intestinal amoebiasis



Entamoeba histolytica

(a protozoan)


  • Ulcers and stools are large, foul-smelling, and brownish black, often with blood-streaked, ‘uncomfortable belly’ or ‘growling abdomen
  • E. histolytica resides in the intestine of an infected individual. Its cyst passes through the stool and contaminates food and water.
  • It spread through contaminated food and water and house flies.

Plague (bubonic, pneumonic and septicemic)

Yersinia pestis (a bacterium)

  • Buboes—swollen, tender lymph nodes, typically found in the armpits and groin.
  • Fever and chills, headache, weakness, fatigue, and internal bleeding.
  • Vector- fleas and rodents


Corynebacterium diphtheria

(a bacterium)

  • A thick grey membrane covers the throat and tonsils, causing sore throat, swollen glands, and fiver.
  • It is airborne

Common cold


  • Infect the nose and respiratory passage but not the lungs, causing nasal congestion and discharge, sore throat, hoarseness, cough, headache, tiredness;
  • Spread through contaminated stuff like cloths


Plasmodium vivax, P. malaria, P. falciparum and P. ovale


  • Periodic chills and fever, anemia, splenomegaly (enlarged spleen), 
  • Vector- female Anopheles mosquito



(nematode or roundworm)

  • Internal bleeding, muscular pain, fever, anemia, and blockage of the intestinal passage;
  • It spreads through contaminated food and water



Lymphatic filariasis

Wuchereria bancrofti, Burgia malayi, and B. timori

(nematode or roundworm)

  • Blockage of lymph nodes causes lymphatic fluid accumulation, resulting in swollen lymph nodes. It causes enlargement of particular body parts, typically limbs and external genitalia. The skin in these areas may thicken and harden. 
  • Vector- Culex quinquefasciatus, Mansonia spp., and Anopheles barbirostris, respectively











  • Slight scaling to blistering and marked disruption of the keratin layer; Dry, scaly lesions;
  • It spreads through contaminated soil, clothing, etc.


Also called

breakbone fever or dandy fever

Dengue virus, DENV (flaviviridae family)

  • Extreme pain and stiffness of joints, Flu-like symptoms, headache, muscle and joint pain, vomiting, rash, hemorrhaging blood vessels,    etc.
  • Vector- Aedes aegypti mosquito


Chikungunya virus, CHIKV (togaviridae family)

  • Fever, headache, rash, joint, and muscle pain
  • Spread by the bite of Aedes aegypti and A. albopictus mosquitoes

Poliomyelitis (Polio)


(Picornaviridae family)

  • The symptoms of fever, headache, nausea, fatigue, and in severe cases, Paralysis.
  • The virus is transmitted from person to person mainly through the fecal-oral route or, less frequently, by a common vehicle (e.g., contaminated water or food)


Clostridium tetani

(a bacterium)

  • Lockjaw (trismus), rigidity, and spasm of voluntary muscles.
  • It can spread by the Clostridium spores present in the soil, which can enter the body through wounds or abrasions

Rheumatoid arthritis

Auto-immune disease

  • It is a chronic, progressive disease
  • Inflammation and thickening of the synovial membranes
  • Swollen and painful Joints

Acquired Immuno Deficiency Disease (AIDS)

Human Immunodeficiency Virus (HIV)

(a retrovirus)

  • Due to Immunodeficiency, any bacterial or viral infection can catch easily, causing Weight loss, fever, flu, etc.
  • Transmission- sexual or body fluid contact


Genetic cause, ionizing radiations, oncoviruses

  • Different symptoms in different types of cancer
  • It may spread from one part of the body to another; this property is called metastasis

Drugs and Alcohol Abuses: An Addiction

Addiction is recognized as a chronic brain disease that involves complex interactions between repeated exposure to drugs (opioids, cannabinoids, coca alkaloids, etc.), alcohol, biological (i.e., genetic and developmental), and environmental (i.e., drug availability, social, and economic variables) factors. Addiction to drugs and alcohol is increasing these days. There are many drugs (legally/illegally) available which is highly addictive. A few drugs, their source, and symptoms of addiction are given in the table below.



Use/Symptoms of addiction


(heroin, fentanyl, oxycodone (OxyContin®), hydrocodone (Vicodin®), codeine, morphine, etc.

Opium poppy,

Papaver somniferum plants

Some are synthetic (fentanyl, oxycodone, hydrocodone codeine)

  • Medically used as an Analgesic (pain reliever).
  • Euphoria, mood changes, drowsiness, mental clouding, respiratory depression, decreased gastrointestinal motility, sedation, nausea, vomiting, constipation, and intestinal bloating.
  • Cardiovascular effects like decreasing blood pressure, causing vasodilation, and decreasing cardiac work.


(Δ-9-tetrahydrocannabinil, marijuana, hashish, charas, and ganja)

Cannabis sativa palnt

  • Medically used as an Antiemetic.
  • Nausea, vomiting, anxiety, fast heartbeat, low blood pressure, muscle relaxation, bloodshot eyes, slowed digestion, dizziness, drowsiness, depression, hallucinations, paranoia.

Coca alkaloid or cocaine

Erythroxylum coca,

Atropa belladonna,

Datura Stramonium Plants

  • Sense of euphoria, increased energy, hallucinations

Barbiturates (Phenobarbital, methohexital, butalbital, pentobarbital, arimidone, amobarbital (all of these are FDA approved)

Chemically derived from barbituric acid

  • Sedative-hypnotic drugs used to treat seizure disorder, insomnia, preoperative anxiety


Derived from the ephedra (Ephedra sinica) plant

  • Used to treat congestion and asthma.
  • Increased blood pressure & pulse rates, insomnia, loss of appetite, and physical exhaustion.
  • Overdose can cause agitation, increased body temperature, hallucinations, convulsions, and possible death


Drug Fact Sheets | DEA.gov

Barbiturates – StatPearls – NCBI Bookshelf (nih.gov)

Erythroxylum Coca – an overview | ScienceDirect Topics

Opioids | National Institute on Drug Abuse (NIDA) (nih.gov)

Disease Definition & Meaning – Merriam-Webster

Disease | definition of disease by Medical dictionary (thefreedictionary.com)

WHO Definition of Disease – Public Health

  • PhD, J. H. E. (2015). Guyton and Hall Textbook of Medical Physiology (Guyton Physiology) (13th ed.). Saunders.
  • Widmaier, E. P., Raff, H., & Strang, K. T. (2003). Vander et al’s Human Physiology: The Mechanisms of Body Function (9th ed.). McGraw-Hill Science/Engineering/Math.

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