Disease and Its Causes

Disease refers to any condition that disturbs or modifies the normal functioning of the living organisms. Each organ system of our body has specific organs to perform particular functions. In the presence of a disease, functioning or the appearance of one or more systems of the body may change. These changes give rise to symptoms and signs of disease.

Symptoms of diseases are the indications that we feel as being wrong or unusual, such as cold, headache, cough, loose motions, etc. These indicate that there may be a disease but, they do not exactly indicate what the disease is. For example, a headache may rarely mean meningitis, it simply means just official stress or any other disease.

Signs of a disease give more definite indications of the presence of a particular disease. They help the doctors (physicians) to diagnose the disease.

Acute and Chronic Diseases
The manifestation of diseases is different depending upon various factors. Depending on the duration of infection, diseases may be of the following two types:

(i) Acute diseases are the diseases that last only for very short period of time. Thus, they do not cause long-term harmful effects on human health.  In these diseases, patients recover completely after the cure, e.g. common cold, cough, etc.

(ii) Chronic diseases are the diseases, which last for a long time, even as much as a lifetime.   They cause drastic long-term effects on human health, e.g. elephantiasis (an infection caused by the filarial worm),   cancer, tuberculosis (T.B), etc.

Chronic Diseases and Poor Health
Acute and chronic diseases have different effects on our health. Any disease that causes poor functioning of one part of the body affects our general health because for general health, all body parts should work properly.  Acute diseases do not have time to cause major effects on general health. Chronic diseases cause major effects because they are long-term diseases. They can cause serious distress to health as compared to acute diseases.

Causes of Diseases
Most of the diseases will have many causes, rather than one single cause.

These causes of diseases are classified into two types:

(i) Immediate causes: These are the real or primary causes also called first level of cause. Various microorganisms like bacteria, virus, fungi, protozoans, etc., that can cause infectious diseases are included in this category.
(ii) Contributory causes: These are also called intrinsic or internal factors. They do not lead to a disease themselves.

These can be of the following types:

(i) Poor health due to inadequate diet: It occurs due to deficiency of one or more nutrients,   making the person unhealthy or susceptible to diseases, e.g. Kwashiorkor, a nutritional deficiency disease occurring due to low protein diet. So, lack of good nourishment becomes the second level of cause of disease.
(ii) Genetic disorders: These are present since birth and pass down from parents to offsprings.
(iii) Lack of public services or poor economic conditions: Poverty also contributes to the cause of disease. These causes become the third level causes of disease.

Infectious and Non-infectious Causes

Immediate causes of diseases belong to two distinct types. One group of diseases is caused by infectious agents like microorganisms.  With the spread of these microbes disease caused by them also spreads. Hence, these diseases are known as infectious diseases.

The second group of diseases is not caused by infectious agents. These diseases are restricted only to the persons, who are suffering from them. As they do not spread from infected person to a healthy person, these are called non-infectious diseases or non-communicable diseases. For e.g., some cancers are caused by genetic abnormalities or by exposure to certain carcinogenic chemicals and radiations. High blood pressure can be caused by excessive weight and lack of exercise, etc.


Infectious diseases are caused by some biological agents (pathogens) such as viruses, bacteria, fungi and single-celled animals, i.e. protozoans, etc.  These can rapidly spread from one person to another by various means such as water, air, food, insects (vectors) or by physical contact.

Infectious Agents
The unicellular or multicellular organisms that cause infection are called infectious agents. These disease-causing organisms are also called pathogens and they are classified into a wide range of categories. Some of them are viruses, bacteria, fungi, protozoans, nematodes, etc.

Various types of pathogen and diseases are as follows:

pathogens and diseases

Note: (i) Viruses, bacteria and fungi multiply very quickly, while worms multiply very slowly in comparison.
(ii) The purpose of dividing infectious agents into different categories is to decide the kind of treatment to use because the same drug will not work against a microbe belonging to a different group.

These are the chemical substances produced mainly by microorganisms (bacteria and fungi). At low concentration they have the ability to inhibit the growth of other microorganisms. They commonly block the biochemical pathways important for bacteria, e.g. penicillin.

The antibiotic penicillin blocks the formation of the cell wall in bacteria. This cell wall helps in the protection of bacteria. Due to penicillin, bacteria become unable to make the cell wall and die easily.

Many antibiotics can work against many species of bacteria rather than simply working against one.

Antibiotics do not work against viral infections because viruses do not use the same pathway as that of bacteria. Therefore, taking antibiotics during common cold does not reduce the severity or duration of disease.


Infectious diseases can spread from an infected person to a healthy person by various ways and therefore, are also called communicable diseases.  Diseases spread through various modes like air, water, sexual contact, vectors, etc. These are discussed below.

(i) Airborne Diseases
Causative microbes of such diseases spread through the air. Little droplets are thrown out by an infected person on sneezing or coughing. These droplets can be breathed in by someone standing close and the microbes get a chance to start new infection. For e.g. common cold, cough, pneumonia and tuberculosis.

Air-transmitted diseases are easy to catch if we are closer to the infected person.  In dosed areas, the droplet nuclei, recirculate and pose a risk for everyone. Overcrowded and poorly ventilated housing is a major factor in spreading airborne diseases.

Air transmitted diseases
Air transmitted diseases

(ii)  Waterborne Diseases
In such diseases, microbes spread through water.  If the excreta from someone suffering from an infectious gut disease, such as cholera,  gets mixed with the drinking water used by people living nearby, it results in waterborne disease. Such diseases are much more likely to spread in the absence of safe supply of drinking water.  For e.g. cholera-causing microbes to enter new hosts through the water they drink and can cause disease in them.

(iii) Sexually Transmitted Diseases (STDs)
These are microbial diseases, that can be transmitted by sexual contact from one partner to the other.  However, STDs do not spread by casual physical contacts. It includes hugs, handshakes, sports such as wrestling or by any other way in which we touch each other socially. Syphilis and AIDS are common examples of sexually transmitted diseases.

Note :  AIDS virus can also spread through blood transfusion. from an infected mother to her baby during pregnancy and through breastfeeding.

Common methods of disease transmission
Common methods of disease transmission

(iv) Spread of Diseases through Vectors
Many animals which live around us in the environment can transmit diseases {germs) from sick person to a healthy person (another potential host).

These animals act as an intermediate and are known as vectors (carrier of the disease or infection), e.g. Mosquitoes such as Anopheles (female)-malaria, Cu/ex-filariasis, Aedes-yellow fever, dengue.

Flies such as housefly-typhoid, cholera, etc.,   tse-tse fly-sleeping sickness, sand-fly, kala-azar.


Immune system is essential for our health. It is the defence system that protects body against various types of microbes. It also keeps the body safe by killing infectious microbes.

When infectious microbes enter the body, the immune system gets activated in response to infection.  An active immune system recruits many cells to the affected area and kills off the disease-causing microbes. This recruitment process is called inflammation. Due to this process, there may be local effects like swelling, pain and general effects like a fever.

Sometimes, allergy (heightened sensitivity to a foreign substance causing body’s defence system to over-react) may also occur in response to allergens. In some cases, the tissue specificity of the infection leads to general seeming effects. For e.g., AIDS virus, HIV reaches the immune system and damages its function.  Due to this body is not able to fight with even minor infections like cold or diarrhoea. This may lead to death.

The severity of disease manifestations indirectly depends on the number of microbes (pathogens) in the body. If the number of microbes is very small, the disease manifestations may be minor or unnoticed. But if the number of microbes is large, the disease can be severe enough to be life-threatening. The importance of the immune system can thus be well-understood from the fact that it is a major factor that determines the number of microbes surviving in the body.


Our body is very large as compared to the microbes. So, they can reach several places in the body through various routes and affect particular tissues or organs. The selection for the site of microbe’s home is related to their point of entry, e.g.

(i) If they enter from the air through no e, they are likely to reach the lungs. This is how tuberculosis-causing bacterium enters the body.
(ii) If they enter through the mouth, they can stay in the gut lining, e.g., typhoid-causing bacteria. They can also go to the liver like jaundice causing virus.
(iii) If they enter through sexual contact, they spread to lymph node all over the body like HIV
(iv) If they enter through mosquito bites, they may enter the liver and then to Red Blood Cells (RBCs) like malaria-causing microbes.  They may reach to the brain like Japanese encephalitis virus that causes brain fever.

Thus, the signs and symptoms of a disease will depend upon the tissue or organ targeted by microbes, e.g.

(i) If the target is the lungs, the symptoms will be a cough and breathlessness.
(ii) If the target is liver, then jaundice may occur.
(iii) If the target is the brain, then headache, vomiting, fits or unconsciousness may occur.
In addition to these organ-specific and tissue-specific effects of infectious diseases, there are other common effects too, which depend on the body’s immune system.


When we get an infectious disease, our doctor may prescribe different medicines for treatment that work differently.

Following are the two ways to treat infectious diseases:

(i) To reduce the effects of die disease: By providing a particular treatment,  so that symptoms can be reduced. The symptoms are usually because of inflammation. For example, we can take medicines that bring down fever, reduce pain or loose motions or take complete rest to conserve energy. But, it will not kill the disease-causing microorganisms. Therefore,  disease would not be cured completely.

(ii) To kill the cause of the disease: Taking medicines that kill microbes is the best way for complete recovery from a particular disease.  It is essential that medicine specific to the disease-causing microbe is taken. For e.g. for the treatment of bacterial disease, antibiotics can be taken by the patient that block essential biochemical pathways of bacteria (these may be pathways for synthesis of new substances or respiration) without affecting our own.  Similarly, there are drugs that can kill protozoans, such as malarial parasite. It is harder to make anti-viral medicines because viruses have very few biochemical mechanisms of their own. They enter our cells and use our machinery for completing their like processes. This means that there are relatively few virus-specific targets to aim at.  However, effective anti-viral drugs that keep HIV infection under control are now available.


Due to modern technology and advancement in science, we have antibiotics and other medicines to cure many diseases. Bur there are few limitations that are normally faced while treating an infectious disease.

These limitations or disadvantages of infectious diseases are as follows:

(i) Once someone has disease, his body functions are damaged and may never recover completely.

(ii) The treatment will take time. It means that someone suffering from a disease is likely to be bedridden for some time even if proper treatment is given.

(iii) The person suffering from an infectious disease can serve as the source for further spread of infection to other people.   Thus, multiplying the difficulties mentioned above. ·

Because of the above limitations, prevention of disease is better than cure.

Methods to Prevent Diseases
There are two general ways to prevent diseases. These are described below:
(i) General Ways of Preventing Infectious Diseases :These ways are mostly related to preventing exposure to infectious microbes. Public hygiene is also necessary for the prevention of infectious diseases.

The practices adopted for the prevention of diseases that are related to the environment are given in the table below:

Some other general principles for prevention are:

(a) Strong immune system: The immune system of our body normally fights against the microbes that enter into our body and kills them.  If the immune system is successful, we do not actually come down with any disease. The cells of the immune system kill off the microbes before it can take on major proportions. As we know, if number of injecting microbes is less, the manifestations of disease will be minor. In other words, becoming exposed to or infected with an infectious microbe does not necessarily mean developing noticeable disease.

(b) Balanced diet: Immune system will not  function properly if sufficient amount of food and nourishment is not available. Therefore, the availability of proper and sufficient food for everyone  is the second basic principle of prevention.

(ii) Specific Ways of Preventing :  Infectious Diseases It includes adopting the practice of immunisation.

This term is derived from Latin word Vaccinia meaning cowpox where the word  Vacca means cow. It is the process of injecting dead microbes in the body of a healthy person in order to develop immunity against a particular disease caused by that microbe.

This eliminates the infection because when the immune system first sees an infectious microbe, it responds against it and then remembers it specifically. So, the body develops a memory of the particular infection by putting something into the body that mimics the microbes we want to vaccinate against.

The vaccine is a preparation of weakened or killed infectious agents or their products. They stimulate the immune system to produce antibodies against particular disease. The  first vaccine was discovered by Edward Jenner against smallpox disease.

Many vaccines are available for preventing infectious diseases and provide disease-specific means of prevention.

The diseases against which vaccines are available are given below:

(i) Diphtheria, Pertussis, Tetanus-DPT (Hib vaccine)
(ii) Poliomyelitis-OPV (Oral Polio Vaccine)
(iii) Hepatitis-B-Hepatitis vaccine
(iv) Tuberculosis-BCG (Bacillus Calmette Guerin)

These vaccines are available under child immunization programme for preventing infectious diseases. Vaccines have to be provided at proper age, e.g. a vaccine  against hepatitis-A is available in the market. Hepatitis virus causes jaundice and transmitted through water. It is useless if its vaccine is given after five years of age because by that time, one has contacted all infection (virus through water) and has become immune to the same.  This is because they are exposed to the virus through the water.

This is the End

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