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The word disease (dis-ease) literally means disturbed ease or uncomfortable. Thus disease can be defined as "Malfunctioning of the body or a part of it due to one reason or the other".
Disease is a condition of the body or a part of it in which functions are disturbed or deranged.

What Does a Disease Look Like?

Symptoms of diseaseSigns of disease
A symptom is a change in normal functioning of  the body which is felt by patient but can not be observed by the doctor, a patient has to tell the doctor about these e.g. stomachache, headache
A sign of a disease is a change in the bodily function or structure that can be observed. e.g. Rashes, swelling of gland

Signs and symptoms help in the diagnosis of a disease. Diagnosis means the process of identifying a disease. In many case doctor also needs the help of laboratory tests of blood, urine, stool etc and diagnostic images like x rays to diagnose a disease.

Pathogens: Disease causing organisms are called pathogens or infectious agents.
Vectors: Animals or insects that carry the infectious agents from one person to another and spread a disease are called vectors.

Insect VectorDisease Transmitted

1. Mosquitoes

(i) Anopheles
 (ii) Culex
 (iii) Aedes

(i) Malaria

(ii) Filariasis

(iii) Yellow fever, Dengue, Filariasis

2. Flies
 (i) House fly
 (ii) Sand fly
 (iii) Tse-tse fly

(i) Thyphoid, Diarrhoea, Dysentry, Cholera,
 Tuberculosis, Conjunctivitis

(ii) Kala azar, Oriental Sore

(ii) Sleeping sickness


3. Louse Epidemic typhus
4. Rat fleaBubonic plague

Types of Diseases | Chemistry for Class 9

Do You Know?


First to look for scientific explanation for diseases. He is remembered


He used non poisonous leeches as an anticoagulant during surgery so
 he is called father of surgery.


He first gave concept of digestive metabolism and immunity. He is
 known as Father of Ayurveda [Ayu- Life, veda- knowledge]

Rudolf Virchow:

Father of Modern pathology.

Father of Immunity:

Edward Jenner (small pox vaccine)

Father of Modern Bacteriology:

Robert Koch (Anthrax T.B., cholera)

Acute and Chronic Diseases

A disease that occurs suddenly and lasts for a short period of time is called an acute disease. 

Example: Common cold, Malaria disease.

A disease that lasts for a long time is called chronic disease e.g. tuberculosis.

Differences Between Acute and Chronic Diseases

Acute DiseaseChronic Disease
Duration: It is of shortor duration.It is longer duration disease.
Persons: Every person suffers from an acute disease at one time or the other.Only some persons suffer from chronic diseases.
Body Damage: Being of short duration, it
 does not damage any organ.
It does damage the affected organ due to prolonged duration.
Recovery: The recovery is generally complete after the treatment.The recovery is seldom complete even after treatment.
Effect: There is neither loss of weight nor
 feeling of weakness.
There is often a loss of weight accompained by feeling of tiredness.
Loss: Interruption of work and loss of efficiency are of short duration.Interrupition of work and loss of efficiency are prolonged.
Example: Diarrhoea, Typhoid.Example: Tuberculosis, Diabetes.

Causes of Diseases

Human health is affected by various factors causes or sources, these factors can be of two major type:

(i) Intrinsic or internal factors: Factors inside the body or within the body.

(ii) External or extrinsic factors: Factors outside the body.

Types of Diseases | Chemistry for Class 9

Classification of some common diseases of human Type of diseasesCauses of diseasesExample
1PhysicalTemprory or permanent damage to body parts.Bone fracture, leprosy
2InfectiousInvasion of the body by other
 organisms (viruses, bacteria, fungi protozoans, nemetodes, insects, etc.)
Typhoid, diarrhoea, malaria, hepatitis, rabies, AIDS, T.B., polio, influenza, cholera.
3DeficiencyInadequate dietKwashiorkor, Marasmus, scurvy, rickets, anaemia, goitre, xerophthalmia, beriberi.
4InheritedDefective genes passed on from
 parents to offspring
Haemophilia, sickle-cell anaemia, cystic fibrosis.
5DegenerativeOrgans and tissues wear away and do not work so well with age.Arthritis, poor sight and hearing defects.
6MentalBrain damage or psychological.Depression, paranoia.
7SocialSocial interactions with family friends and strangers.Drug dependence alcoholism, smoking, unhealthy life, social isolation.

Means of Spread

Disease causing microbes can spread through the air. This occurs through the little droplets thrown out by an infected person who cough or sneezes. Disease can also be spread through water. When infectious agents get mixed with drinking water.

Some diseases are spread by sexual contacts or blood transfusions animals can also carry infectious agents.

Organ-specific and Tissue-specific Manifestations

Different species of microbes enters through various points of body and there are many possible places organs or tissues where they could go.

If they enter from the air via nose, they are likely to go to the lungs. If they enter through the mouth they can stay in gut lining.

Malaria causing microbes enter through a mosquito bite and will go to the liver and then to Red blood cells but virus of Japanese encephalitis or brain fever will similarly enter through mosquito bite will enter in brain or goes to infect brain.

The signs and symptoms of a disease will thus depend on the tissue or organs which the microbes target.

Bacterial disease

Types of Diseases | Chemistry for Class 9

Types of Diseases | Chemistry for Class 9

Types of Diseases | Chemistry for Class 9

Food poisoning by bacteria Salmonella is called salmonellosis. BacteriaSalmonella is present in the sick farm animals. So the infection of bacteriaSalmonella which causes food poisoning is contracted from the sick farm animals through their contaminated food products such as milk, eggs and meat.

Toxins released by Escherichia coli cause mild diarrhoea to severe dehydration.Shigellosis, caused by Shigella, is characterised by frequent passage of stools with blood and mucus and abdominal cramps.

Clostridium botulinum causes food poisoning or botulism from taking preserved foods (dibba-bund food).

Viral disease

1Polio/PoliomyelitisPolio virusFever, headache, Paralysis Prevention by
 OPV - ORAL Polio vaccine.
2InfluenzaOrthomyxovirus or Influenza virusSneezing, fever, headache. Muscular Pains, Coughing. Discharge from nose, Antibiotic
 suggested by doctor.
3MeaslesParamyxovirusHigh grade fever, white brown patches on
 body and blisters Antibiotics and M.M.R.
4Chicken Pox

Pox virus/ varicella 

herpes virus

Rashes on body with fever (Rashes are dewdrop like in appearance)
5M U M P SParamyxovirusPainful swelling in Parotid gland
 Anti biotics - Mumps vaccine

Dengue fever or

Break bone fever

Arbovirus flavi

vector Aedes egypti

Fever, Pain in muscles and joints

hemorrhagic condition in body.




Rhabdovirus or street virus 

vector- Dog, cat, wild animal

Affect CNS- Madness hydrophobia due to laryngeal spasm and 100% death occur
 vaccine - Human diploid cell culture vaccine.

Protozoan Disease

1MalariaPlasmodium SPSHigh fever with chill of intermittent
 Periodically Pain in joints - Quinine
2AmoebiasisEntamoeba histolyticaIntestinal spasms dysentry Antibiotics
3African sleeping sicknessTrypanosoma gambiensPatient feels sleepy, nervous system impairment
4Kala azarLeishmania donovaniHigh fever associated with enlargement
 of spleen and liver
5DiarrhoeaGiardia intestinalisVomiting loose motions 

Types of Diseases | Chemistry for Class 9

Types of Diseases | Chemistry for Class 9

Cancer (crab = cancer)

Uncontrolled , abnormal and excessive mitotic division of cells is called cancer
Oncology: Study of cancer is called oncology.
Cancerous cells: The abnormal and undifferentiated cells are called cancerous cells.
Cancer is a non infections disease but cancer cells spread from one part to another part of body through blood lymph etc.
Tumour or Neoplasm: The abnormal tissue that grows continuously forms a tumour. Tumour's are of two types (1) Benign, (2) Malignant
Carcinogens: The factor that induce cancer are called carcinogens they include (i) Pollutants (ii) Radiations (iii) Tobacco (iv) Biological agents (v) Mechanical agents.

Symptoms: Initially a lump or swelling or sore development followed by unexplained loss of weight, difficulty in swallowing, Persistent cough etc.
Types of cancer depending on the organs affected

(i) Carcinoma: In skin glands, lungs, breast, pancreas and stomach

(ii) Sarcoma: In connective tissues, bones, muscles.

(iii) Leukemia: Increase in no. of white blood cells.

Metastasis: The stage of cancer in which it becomes malignant and spreads to different body is called metastasis.

Diagnosis can be done by endoscopy and biopsy

Cure: (i) Radiation therapy (ii) Chemo therapy (iii) Surgery


Types of Diseases | Chemistry for Class 9

AIDS: Acquired immuno deficiency syndrome.
Casual Agent: Human immuno deficiency virus H.I.V. This virus belong to retrovirus group of viruses.

HIV gradually reduces the efficiency of human immune system. This make the body vulnerable to other life threatening disease that finally causes patient's death.

How does a person get infected with HIV?

This virus is transmitted through blood, semen and breast milk. Thus a person can get HIV by:

(i) Transfusion of infected blood.

(ii) Sharing of needle with an infected person.

(iii) Having sexual intercourse with an infected person.

(iv) An unborn child can get infection from the mother.

(v) Infected mother's milk can infect an infant.

HIV is not contracted by touching an infected person nor can it be contracted by breathing in the virus when patient coughs or sneeze.

Signs and symptoms of AIDS: (i) Swollen lymph glands (ii) Weight loss (iii) Frequent fever (iv) Night sweat (v) Diarrhoea

The disease may affect the central nervous system which may cause memory loss and difficulty in speaking.

Prevention and control: No drug has been developed to cure AIDS only prevention is cure. A patient is usually given a combination of drug that manage to slow down the progress of that disease. Scientists are working on drugs that promise to cure this disease in the near future.

1. For blood transfusion always use blood which has been screened for HIV.

2. Use of disposable needles and syringes blade or razors

3. Avoid multiple sex partnership

4. Condoms can be used to prevent the transmission of HIV through sexual fluids.

The drug zidovudine (AZT) stops HIV replicating by binding with reverse transcriptase enzyme and blocking its action. But it has a side effect, i.e., it causes anaemia.

Jaundice / HEPATITIS

Hepatitis is a disease in which liver became enlarged.


(i) Yellowing of skin and whites of eyes This condition referred to as jaundice

(ii) Dark urine

(iii) Whitish Stools

Note: The yellowing is because of the presence of excessive bilirubin (a bile pigment) in the blood.

Viral hepatitis is caused by different strains of hepatitis virus. Hepatitis caused by a virus is named after it. Thus we have hepatitis A, hepatitis B....... hepatitis G.Named after the strains ABCDEF and G.

Transmission of HEPATITIS

(i) Some strains of Hepatitis like hepatitis A is transmitted through contaminated food or water.

(ii) Hepatitis B ® Through blood, semen, saliva, breast milk.

General Hepatitis patient need 4 weeks to recover but certain types of hepatitis can cause death.

Prevention and Control : A hepatitis patient is advised to take light food with high carbohydrate and low fat to give rest to the liver An antiviral injection (interferon) controls disease. The patient should take bed rest.

Good sanitation prevent the spread of disease by food and water use clean and hygienic water for drinking. Tested blood should be used for transfusion. use of disposable needle and syrings.

vaccine like A- vaccine, Engerix B, Shenva-B, Enivac-B, Provides immunity against hepatitis A and B.

Types of Diseases | Chemistry for Class 9

Hemophilia is sex linked hereditary disease. Genes of this disease found on sex chromosomes.
Characters: In case of disease blood clots very slow so patient die due to excess of hemorrhage.

Hemophilia is caused by a recessive gene due to presence of two x chromosome. The disease is not developed in woman inspite of presence of gene for hemophilia on x chromosome. This is due to suppression of expression of recessive gene in presence of dominant gene.

While in man the recessive gene can be expressed if present on x chromosome. Due to presence of only one x chromosome and y chromosome in the genotype. Women is known as a carrier, she herself does not show the symptoms but pass the gene to off spring.

Colour blindness/ Daltonism
It is a recessive sex linked defect in which human beings are unable to distinguish red and green colour. Its genes are also situated on sex chromosome X. In colour blindness womens are found generally as carrier and men as patients. Heredity of colour blindness is similar to that of hemophilia.

Types of Diseases | Chemistry for Class 9


It is a genetic disease that leads to synthesis of defective subunits of haemoglobinBlood of a sufferer child have to change frequently. Cause of this disease is a recessive gene. only a homozygous child with recessive gene shows the symptoms and heterozygous remains normal.

Antibiotics and Interferon

Antibiotics are chemical substances produced by living organisms such as bacteria and fungi, etc., which can kill or stop the growth of some pathogenic (disease producing) microorganism such as bacteria and fungi. Some of the important antibiotics which are used for the treatment of disease caused by bacteria and fungi : 1. Penicillin; 2. Tetracycline; 3. Streptomycin; 4. Chloromycetin; 5. Quinclones; 6. Cephalosporin; 7. Gentamicin; 8. Chloramphenicol and 9. Rifampicin.

The treatment of viral infections is more difficult than that of bacterial infection. For each type of viral infection, human body produces its own antiviral protein, called interferon (IFN). Interferon 

is released from the infected and dying host cells. On reaching the un infected cells, IFN makes them resistant to the virus infection. Interferon have been proved to be effective in treating influenza and hepatitis.

Immunization: Stimulating the body to produce antibodies by artificial means is known as Immunization.

For Competitive Exams

Helminth Disease


Ascaris lumbricoides 

loosemotion, restlessness.

Abdominal spasm, insomia, vomiting
2DracunculiasisDracunculus medinensisBlisters on skin of arms shoulder and legs
3Elephantiasis or filariasisWuchereria bancroftiSwelling of hand scrotum testis and breasts
4TaeniasisTaenia soliumAbdominal pain, nausea, Anemia, loss of appetite, indigestion, nervous disorders

Types of Diseases | Chemistry for Class 9Types of Diseases | Chemistry for Class 9

Non Communicable Diseases

Types of Diseases | Chemistry for Class 9

S.NO.CauseName of disease due to deficiencySymptoms of Disease

Protein energy 

Malnutrition (PEM)


Infants - below 1 year age

Shrivelled appearance, thinning of limbs Prominent ribs, retarded physical and
 Mental growth. digestive disorder,
 repeated diarrhoea

Protein (PEM) 


Kwashiorkor (1–3 year age)loss of appetite stuntedgrowth, bulging eyes, enlarged stomach, long, thin and curved legs.
3IronAnaemiaBecomes pale, loose appetite feels tired

Vitamin B12 

(cyano cobalamine)

Pernicious anaemiaPerson become pale, loss of appetite, feels tired reduced number of RBC
5Folic acidMacrocytic anaemiaBecome pale, loose appetite, feels tired
 reduced growth of RBC
6IodineGoitreAbnormal enlargement of thyroid, swelling
 in the neck reduced thyroid function,
 retarded growth.
7Vitamin A (Retinol)XeropthalmiaNight blindness dryness of cornea

Vitamin D


Rickets [Children]

osteomalacia adults

Twisted, thin, soft, bent bones, deformed ribs

Vitamin B1


Beri-BeriMuscular dystrophy, Nervous disorder

Vitamin B3


Pellagra4D' syndrome (dermatitis, diarrhoea, dementia death)

Vitamin C

(Ascorbic acid)

ScurvyBleeding gums, loosening and falling of teeth, loss of weight. 

Types of Diseases | Chemistry for Class 9

Types of Diseases | Chemistry for Class 9

Types of Diseases | Chemistry for Class 9

Degenerative Diseases


Heart attack

Liver failure
 Kidney failure



Liver megaly

2Hyper sensitivityAllergySneezing, coughing, watering of eyes
 asthma, hay fever.


growth of cells

Cancer and tumourloss of weight, person become weak can lead to death or destruction of affected organ.
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