Common Disease in human


Any functional or physical change from the normal state that causes discomfort, or disability, or impairs the health of a living organisms is called a disease. In other words, malfunctioning of the body or part of the body  with specific  symptoms is known as disease

Theory : Health  does not mean " absence of disease or physical fitness". Health may be defined as state of compete physical,mental and social well being and not only absence of disease

Types of Diseases

The diseases can be broadly grouped into three types :Communicable ( infectious), non_ communicable (non_ infectious) diseases and congenital diseases (genetic disorders). Communicable and non_ communicable diseases develop after birth, therefore,  they are acquired diseases while in congenital diseases anatomical or physiological abnormalities are present from birth.

1) Communicable (infectious) diseases:

These diseases are caused by pathogens such as viruses, bacteria,Rickettsias, Chlamydiae, fungi,  protozoans, flat worms and round worms. Infectious diseases are very common. Some of the infectious diseases like AIDS are fatal. These diseases are spread  to other persons. 

2) Non communicable(Non infectious)diseases :

These diseases are not spread to other persons. Among non_ infectious diseases cancer is the major cause of death. These  are of four types.

(a) Deficiency Diseases: These diseases are due to deficiency of nutrients and hormones, e.g.,marasmus, kwashiokar, beri beri, scurvy, diabetes, etc.

(b) Degenerative Diseases: These diseases are due to malfunctioning of some important organs, e.g., heart diseases are due to abnormal working of some body parts.

(c) Allergic Diseases: These diseases are caused when the body becomes hypersensitive to certain foreign substances.Asthma is an allergic disease. 

(d) Cancer : It is caused  due to uncontrolled growth of certain  tissues in the body. 

3) Congenital Diseases ( Genetic Disorders) :

As stated earlier these are found  from birth. They may be caused by 

(i) A single  gene mutation, e.g., haemophilia, colour blindness, phenlketonuria, alkaptonuria, albinism, sickle cell anaemia,

(ii) Chromosomal  aberrations, e.g., Down's syndrome, Turner's syndrome, Klinefelter's syndrome, and 

(iii) Environmental factors, e.g,. harelip, cleft palate. The gene and chromosome induced congenital defects are transmitted to the next generation, however, abnormalities caused by environmental factors are not transferred to the children.

Disease Agents

The disease agent is a substance or a force which causes a disease by its excess or deficiency or absence. These agents are of five main types. 

1) Biological Agents or Pathogens 

(e.g., viruses, bacteria, mycoplasma, fungi, protozoans, helminths, insects and other organisms).

2) Nutrients Agents 

(e.g., minerals, carbohydrates, proteins, fats, vitamins and water).

3) Chemical Agents :

These are of two types .(i) Endogenous chemical Agents (e.g,. urea, uric acid, hormones and enzymes).

(ii) Exogenous  Chemical Agents (e.g., gases, dusts, metals, fumes, etc. and allergens like spores and pollen).

4) Physical  Agents 

(e.g., heat, cold, humidity pressure, radiation, electricity and sound).

5) Mechanical Agents 

(e.g., injuries, fractures, sprains and dislocations).

Transmission of Diseases (Pathogens) 

1) Direct Transmission: The pathogens of some diseases are carried directly to the human body without intermediate agents. It occurs in many ways :

(i) Contact with Infected Persons (e.g., Chicken pox, small pox, Syphilis, et. 

(ii) Droplet infection  by sneezing, coughing, spitting and talking (e.g., diphtheria, influenza, common cold, measles, tuberculosis, Pneumonia, etc). 

(iii) Contact with Soil (e.g., bacteria responsible for tetanus).

(iv) Animal Bites (e.g., rabies bite).

(v) Through Placenta (e.g., German measles and bacteria of syphilis can be carried from mother of foetus via placenta).

2) Indirect Transmission: The pathogens of some diseases are carried through some intermediate agents.It occurs in the following ways.

(i) Arthropod Vectors (e.g., House fly__ typhoid, cholera, dysentery, blood sucking insects, female Anopheles _ malaria, Tse tse fly__ African sleeping sickness).

(ii) Vehicle __ borne Method (e.g., cholera, dysentery, typhoid, AIDS and Hepatitis B).

(iii) Air _ borne Method (e.g., epidemic typhus).

(iv) Formite__ born Method (e.g., influenza, cholera etc).

(v) Human Carriers (e.g., diphtheria and typhoid).

How pathogens cause Diseases)

Pathogens causes diseases in two ways : tissue damage  and toxin secretion. 

1) Tissue Damage : The virus of rabies destroys brain tissue. The bacteria of pulmonary tuberculosis damages the ces of lungs.

2) Toxin Secretion  : Many microorganisms ( pathogens) produce powerful poisons called toxins, which cause diseases. Toxins are of two types :
(i) Exotoxins : These toxins are released as soon as secreted. The diseases caused by exotoxins include diphtheria, tetanus, etc.

(ii) Endotoxins : These toxins are retained in the bacterial cells and released when bacteria die and disintegrate. The diseases brought about  by endotoxine include dysentery, cholera, typhoid fever and bubonic plague.

Most dangerous disease in human body 

The most dangerous disease in the human body is a subjective question as it can vary depending on various factors such as severity, mortality rate, and global impact. However, some of the deadliest diseases include:

1. Ebola: Ebola virus disease (EVD) is a highly contagious and often fatal illness that causes severe hemorrhagic fever.

2. HIV/AIDS: Human Immunodeficiency Virus (HIV) attacks the immune system, leading to acquired immunodeficiency syndrome (AIDS), which leaves individuals susceptible to various infections and diseases.

3. Smallpox: Although it has been eradicated through widespread vaccination efforts, smallpox was a highly lethal viral disease that caused significant morbidity and mortality.

4. Rabies: Rabies is a viral disease transmitted through animal bites, causing inflammation of the brain and spinal cord. If left untreated, it is nearly always fatal.

5. SARS-CoV-2 (COVID-19): The ongoing COVID-19 pandemic caused by the novel coronavirus has had a significant global impact, leading to millions of infections and deaths worldwide.

6. Hemorrhagic fevers (e.g., Ebola, Marburg, Lassa fever): These viral infections cause high fever, accompanied by bleeding disorders, organ failure, and a high mortality rate.

7. Hantavirus Pulmonary Syndrome: This rare but severe respiratory disease is caused by infection with hantaviruses, which primarily spread through contact with infected rodents.

It's important to note that this list is not exhaustive, and diseases' dangerousness can vary based on different criteria, including region, healthcare access, and individual health conditions. It is essential to continue practicing preventive measures and seek medical care when needed to prevent and manage these diseases effectively.

Human Health 

Health, for a long time, was considered as a state of body and mind where  there was a balance of certain 'humors'. This is what early Greeks like Hippocrates as well as Indian Ayurveda system of medicine asserted .It was throught that persons with 'blackbile' belonged to hot personality and would have fevers. This idea was arrived at by pure reflective thought. The discovery of blood circulation by William Harvey using experimental method and the demonstration of normal body temperature in persons with blackbile using thermometers disproved the ' good humor'hypothesis of health. In later years, biology stated that mind influences, through neural system and endocrine system, our immune system and that our immune system maintains our health. Hence, mind and mental state can affect our health. Of course, health is affected by__ 

(i) genetic disorders __ deficiencies with which a child is born and deficiencies / defects which the child inherits from parents from birth.

(ii) infections and 

(iii) Life style including food and water we take, rest and exercise we give to our bodies, habits that we have or lack etc.

 The term health is very frequently used by everybody.How do we define it ?Health does not simply mean ' absence of disease'or ' physical fitness '. It could be defined as a state of compete physical , mental and social well _ beings.When people are healthy, they are more efficient  at work. This increases productivity and brings economic prosperity.Health also increases longevity of people and reduces infant and maternal mortality. 

    Balanced diet, personal hygiene and regular exercise are very important to maintain good health. Yoga has been practised since time immemorial to achieve physical and mental health. Awareness about diseases and their effect on different bodily  functions, vaccination (immunisa__   tion) against infectious diseases,  proper disposal of wastes, control of vectors and maintenance of hygienic food and water resources are necessary for achieving  good health. 

   When the functioning of one  or more organs or systems of the body is adversely  affected, characterised by various signs and symptoms, we say that we are not healthy, i.e., we have a disease. Diseases can be broadly grouped into infectious and non_ infectious. Diseases which are easily transmitted from one person to another, are called infectious diseases. Infectious diseases are very common and very one of us suffers from these at sometime or other.Some of the infectious diseases like AIDS are fatal. Among non__ infectious diseases, cancers is the major cause of death.Drug and alcohol abuse also affect our health adversely.

Common Diseases in Humans 

A wide range of organisms belonging to bacteria, viruses, fungi, protozoans,  helminths, etc., could cause diseases in man. Such disease causing organisms are called pathogens. Most parasites are therefore pathogens as they cause harm to the host  by living  in (or on) them.The pathogens can enter our body by various means, multiply and interfere with normal viral  activities,  resulting in morphological  and functional damage. Pathogens have to adapt to life within the environment of the host. For example, the pathogens that enter the gut must known a way of surviving in the stomach at low pH and resisting  the various digestive enzymes. A few  representative members from different groups of pathogenic organisms are discussed here alongwith the diseases caused by them. Preventive and control measures against these diseases in general are also briefly  described. 

    Salmonella typhi is a pathogenic bacterium which causes typhoid fever in human beings. These pathogens generally enter the small  intestine through food and  water contaminated with them and migrate to other organs through  blood. Sustained high fever (39° to 40°C), weakness, stomach pain, constipation, headache and loss of appetite are some of the common symptoms of this disease. Intestinal perforation and death may occur in severe cases. Typhoid fever could be confirmed by Widal test. A classic case in medicine, that of Mary Mallon nicknamed Typhoid Mary, is worth mentioning here. She was a cook by profession and was a typhoid carrier who continued to spread typhoid for several years through the food she prepared. 

    Bacteria like Streptococcus pneumoniae and Haemophilus influenzae are responsible for the disease pneumonia in humans which infects the alveoli (air filled sacs) of the lungs. As a result of the infection,  the alveoli get filled with fluid leading to severe problems in respiration. The symptoms of pneumonia include fever, chills, cough and headache.In severe cases, the lips  and finger nails may turn gray to bluish in colour. A healthy person Acquires the infection by inhaling the droplets / aerosols released  by an infected person or even by sharing glasses and  utensils with an infected person. Dysentery, plague, diphtheria, etc., are some of the other bacterial diseases in man.

Cause Diseases in Human Beings:

  Many viruses also causes diseases in human beings.Rhino viruses represents one such group of viruses which cause one of the most infectious human ailments __ the common cold. They infect the nose and respiratory passage but not the lungs. The common cold is characterised by nasal congestion and discharge, sore throat, hoarseness, cough, headache, tiredness, etc. which usually last for 3_ 7 days. Droplets resulting from  cough or sneezes of an infected person are either inhaled directly or transmitted through contaminated objects such as pens, books , cups, doorknobs, computer keyboard or mouse, etc., and cause infection in a healthy person .

  Some of the human diseases are caused by protozoans too.You might have heard about malaria, a disease man has been fighting since many years. Plasmodium, [ P.vivax, P.malaria and p. Falciparum] are responsible for different types of malaria. Of these, malignant malaria caused by Plasmodium falciparumis the most serious one and can even be fatal.

Stages in the life Cycle of Plasmodium :

  Let us take a glance at the life cycle of Plasmodium .

Plasmodium enters the human body as spirozoites ( infectious form) through the bite of infected female Anopheles mosquito. The parasites initially multiply within the liver cells and then attack the red blood cells ( RBCs) resulting  in their rupture. The rupture of RBCs is associated  with release of a toxic substance, haemozoin, which is responsible  for the chill and high fever recurring every three to four days .When a female Anophelses mosquito bites an infected development. The parasites enter the mosquito's body and undergo further development. The parasites multiply within them to form sporozoites that are stored in their salivary glands. When these mosquitoes bite a human, the sporozoites are introduced into his her body, thereby initiating the events mentioned above. It is interesting to not that the malarial parasite requires two hosts__ human  and mosquitoes __ to complete its life cycle ; the femal Anopheles mosquito is the vector ( transmitting agent) too.

  Entamoeba histolyticais a protozoans parasite in the large intestine of human which causes amoebiasis ( amoebic dysentery).Symptoms of this disease include constipation, abdominal pain and cramps, stools with excess mucous and blood clots. Houseflies act as mechanical carriers and serve to transmit the parasite from faeces of infected person to food and food products, thereby contaminating them. Drinking water and food contaminated by  the faecal matter are the main source of infection. 


Ascaris, the common round worm and Wuchereria, the filarial worm, are some of the helminths which are known to be pathogenic to man. Ascaris, an intestinal parasite causes ascariasis.Symptoms of these disease include internal bleeding, muscular pain, fever, anemia and blockage of the intestinal passage. The eggs of the parasite are excreted along with the faeces of infected persons which contaminate soil, water, plants, etc.A healthy person acquires this infection through contaminated water, vegetables, fruits, etc.


Wuchereria ( W.bancroftaind W. malayi), the filarial worms cause a slowly developing chronic inflammation of the organs in  which they live for many years, usually the lymphatic vessels of the lower limbs and the disease is called elephantiasis or filariasis .The genital organs are also often affected, resulting in gross deformities. The pathogens are transmitted to a healthy person through the bite by  the female mosquito vectors. 


Many fungi belonging to the genera Microsporum, Trichophyton and Epidermophyton are responsible for ringworms which is one of the most common infectious diseases in man. Appearance of dry, scaly lesions on various parts of the body such as skin, nails and scalp are the main symptoms of the disease. These lesions are accompanied by intense itching. Heat and moisture help these fungi to grow, which makes them thrive in skin folds such as those in the groin or between the toes.Ringworms are generally acquired from soil  or by using towels, clothes or even the com of infected individuals. 


Maintenance of personal and public hygiene is very  important for prevention and control of many infectious diseases. Measures for personal hygiene include keeping the body clean; consumption of clean drinking water, food, vegetables, fruits, etc. Public hygiene includes  proper disposal of waste and excrete; periodic cleaning and disinfection of water reservoirs, pools cesspools and tanks and observing standard practices of hygiene in public catering. These measures are particularly essential where the infectious agents are transmitted through food and water such as typhoid, amoebiasis and ascariasis. In cases of air_ borne diseases such as pneumonia and common cold, in addition to the above measures, close contact with the infected persons or their belongings should be avoided. For  diseases such as malaria and filariasis that are transmitted through insect vectors, the most important measure is to control or eliminate the vectors and their breeding places. This can be achieved by avoiding stagnation of water in and around residential areas, regular cleaning of household coolers, use of mosquito nets, introducing fishes like Gambusia in ponds that  feed on mosquito larvae, spraying of insecticides in ditches, drainage areas and swamps, etc.In addition, doors and windows should be provided with wire mesh to prevent the entry of mosquitoes. Such precautions have become all the more important especially in the light of recent widespread incidences of the vector__ borne ( Aedes mosquitoes) diseases like dengue and Chikungunya in many parts of India.


The advancement made in biological science have armed us to effectively deal with many infectious diseases. The use of vaccines and immunisation programmes have enables us to completely eradicate a deadly disease like smallpox. A large number  of other  infectious  diseases like polio, diphtheria, pneumonia and tetanus have been controlled to a large extent by the use of vaccines. Biotechnology is at the verge of making available newer and safer vaccines. Discovery of antibiotics and various other drugs has also enabled us to effectively treat infectious diseases. 


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