Diseases caused by Microorganisms, Diseases in Plants and Animals due to Microorganisms, Food Poisoning and Preservation- Biology Guide for Class 8

Information about Diseases caused by Microorganisms


Diseases caused by Microorganisms, Diseases in Plants and Animals due to Microorganisms, Food Poisoning and Preservation


Class 8


Class 8 Biology

Topics Covered

  • Microorganisms causing diseases in Humans
  • Prevention of Diseases
  • Microorganisms causing Diseases in Animals
  • Microorganisms causing Diseases in Plants
  • Food Poisoning
  • Food Preservation

Some microorganisms are harmful in different ways. They cause diseases in human beings, plants and animals. Such disease causing organisms are known as pathogens.
Some microorganisms also cause spoilage of food. Some grow on leather and clothes and have an adverse affect on their quality.
  • Bacteria also grow on food particles that may get lodged in between our teeth. Acids produced by these bacteria, corrode the tooth enamel and cause cavities.
  • Many microorganisms grow in water bodies and decrease their oxygen level. This is harmful to the other organisms living in these water bodies.

Microorganisms causing diseases in Humans

  • During our lifetime, we come in contact with many microorganisms. Some of these microorganisms leave undesirable effects on our body.
  • They invade our body and multiply inside it.
  • They release some harmful materials, called toxins, in our body.
  • These toxins adversely affect our body and can make us suffer from diseases.

Communicable diseases

  • Diseases, which can spread from an infected person to a healthy person, are called communicable diseases.
  • Communicable diseases spread from one person to another through air, water, food, physical contact or insects.
  • In diseases, like tuberculosis and pneumonia, when a patient sneezes or coughs, small droplets carrying germs, are released in the air. These germs, if inhaled, can infect a healthy person.
  • The consumption of contaminated food can result in diseases like cholera, typhoid or hepatitis.
  • Different types of insects can also spread various diseases. For example, malaria is spread by the female Anopheles mosquito, dengue by Aedes mosquito and plague by rat flea.
  • Diseases, like common cold and conjunctivitis, can spread through direct, or indirect, contact with a patient.

Some common human diseases and the way they get transmitted are listed below.

Infectious Diseases Caused by Microorganisms


Diseases Caused


Tuberculosis, diphtheria, cholera, tetanus, typhoid


Common cold, influenza, mumps, polio, chickenpox, AIDS


Ringworm, athlete's foot


Malaria, amoebic dysentery, sleeping sickness

Modes of Transmission of Pathogens

Modes of Transmission

Related Diseases

Air (actions like sneezing, coughing)

Tuberculosis, common cold, influenza, swine flu

Water (using contaminated water)

Typhoid, amoebic dysentery, cholera

Soil (consuming food items that are not washed/cleaned properly; wounds)


Animals (direct contact (e.g. being bitten by a rabid dog); through vectors that spread disease)

Rabies, malaria

Prevention of Diseases

Various ways that can help in preventing diseases are given in the following table:

Methods of prevention

Diseases that may be prevented


Tuberculosis, polio, mumps, tetanus

Using mosquito nets and mosquito repellents, controlling mosquito population

Malaria, dengue

Proper disposal of waste and using disinfected water


Maintaining good personal hygiene

Ringworm and athlete's foot

Microorganisms causing Diseases in Animals

Microorganisms also cause diseases in animals. We have heard about rabies (a disease that affects animals like dogs), and foot and mouth disease (that affects cattle).
Such diseases may affect domestic animals. Pets and human beings may also get infected if they come in contact with the infected animals.

The following table tells us about some diseases caused by the microorganisms in animals.

Name of the disease

Microorganisms that cause the disease

Animals infected




Foot and mouth diseases





Dogs, monkeys



Cattle, poultry



Cattle, poultry




Canine distemper



Microorganisms causing Diseases in Plants

We have observed some plants with wrinkled leaves, or plants and trees having an abnormal branching pattern or having abnormal fruits.
  • These plants may have been affected by a pathogen. Most plant diseases are caused by fungi, bacteria and viruses.
  • These microorganisms may reduce crop yield and, at times, cause total destruction of the crops.
  • The Irish Famine in 1740-41 was caused because of complete destruction of the potato crop due to the disease, 'late blight', caused by a fungus.
The following table gives information about some common plant diseases that are caused by different microorganisms.


Related Plant disease


Tobacco mosaic virus (appearance of uneven spots and discolouration on the leaves)


Citrus canker (lesions on leaves, stems and fruit, with raised brown water soaked margins)


Rust of wheat (diseased plants show rust-coloured orange patches on the infected plant parts)


Smut of rice (smut balls in grains that become greenish black; then burst and infect other grains)


Red rot of sugarcane (the infected stems have a dull red colour interrupted by occasional whitish patches across the stalk, and/or elongated red lesions on the midribs of leaves)

Food Poisoning

  • Food poisoning is a disease that results from the consumption of contaminated food.
  • Symptoms of food poisoning are vomitting, nausea, severe pain in the abdominal region, diarrhoea and dehydration.
  • It may also cause weakness, fatigue and damage to the nervous system. These symptoms arise due to the presence of bacteria, or other microbes, in food.
  • They may also be due to ingestion of toxins contained in food (including those produced by bacteria). Some bacteria, like Clostridium and Staphylococci, and fungi, like Aspergillus, cause food poisoning. 
  • Food must be properly prepared and stored to prevent food poisoning.
  • Food poisoning can occur when food is left unrefrigerated for long periods of time.
  • This often happens at picnics and large parties.
  • Sometimes, mishandling also causes food poisoning. Persons, who handle or prepare food, should wash their hands to prevent contamination of food. 

Food Preservation

  • Food preservation is the process of treating and handling food so as to stop, or greatly slow down, spoilage (loss of quality, edibility or nutritive value) caused, or accelerated, by microorganisms.
  • Preservation usually involves preventing the growth of bacteria, fungi and other microorganisms, as well as retarding the oxidation of fats which cause rancidity.
  • Rancidity means having disagreeable odour, or taste due to decomposition of oils or fats.
  • Some preservation methods require the food to be sealed after treatment to prevent recontamination with microbes.
  • Other methods, such as drying, allow food to be stored without any special containment for long periods.
 A brief description of some of the methods used for 'food preservation' is given below. 

1. Drying

  • This method reduces water content sufficiently and, thereby, prevents, or delays, bacterial growth.
  • Drying also reduces weight, making food more portable.
  • Some common food stuffs that are preserved by drying are: apples, pears, bananas, mangoes, papaya, apricot and coconut. Drying is also the normal means of preservation for cereals (grains), such as wheat, maize, oats, barley, rice, millet and rye.

2. Preservation using sugar and salt

  • Sugar and salt reduce the water content and make it unavailable for the growth of microorganisms. As a result, the food gets preserved.
  • Meat and fish are covered with salt to check bacterial growth.
  • Some pickles are also preserved by the addition of specific quantities of salt.
  • Jams and jellies are usually preserved by addition of sugar.

3. Preservation using acetic acid

  • Acetic acid, in the form of vinegar, is used in the manufacture of several pickled products.
  • Vinegar also stops the growth of microorganisms.
  • Citric acid and phosphoric acid are also used, in carbonated beverages and fruit drinks, for both flavouring and preservation.

4. Preservation using chemical preservatives

  • Chemical preservatives, like sodium benzoate and potassium metabisulfite are used to preserve jams, jellies and pickles. These chemicals inhibit the growth of microorganisms.

5. Heat and cold treatments

  • Some food items, like milk, are usually boiled, before their use, or storage.
  • Boiling kills many microorganisms.
  • Several food stuffs are also stored in the refrigerator; the low temperature inhibits the growth of microorganisms.

6. Pasteurisation

  • Pasteurisation is 'pressurised heating' for a short time; it may be considered as a mild form of heat treatment. The temperature, used during pasteurisation, is below 212°F (100°C).
  • Milk is pasteurised to destroy microorganisms. However, there are many more heat-resistant organisms in it that only get reduced in number. Hence, pasteurised milk needs to be stored under refrigeration to keep bacterial growth in check.
  • In addition to destroying some microorganisms, pasteurisation also inactivates some enzymes; that, at times, can be a disadvantage.

7. Vacuum packing

  • Vacuum packing stores food in a vacuum environment; usually an air-tight bag or bottle.
  • The vacuum environment deprives bacteria of the oxygen needed for their survival; it, therefore, slows 'spoiling'.
  • Vacuum packing is commonly used for storing nuts; it helps to reduce their loss of flavour caused by their oxidation.

8. Canning

  • It involves cooking food and sealing it in sterile cans or jars; this is followed by boiling the containers to kill or weaken any remaining bacteria (a form of sterilisation).
  • However, food preserved by canning, or bottling, is at immediate risk of spoilage once the can or bottle has been opened.
  • At times, the cans or jars get damaged and show puffing or enlargement. This indicates spoilage of food; the food, in such puffed or enlarged containers, should never be consumed.

9. Freezing

  • When foods are kept at below freezing temperatures, most chemical changes take place at such a reduced rate that only minor changes are noticeable even after long periods of storage.
  • However, microorganisms are generally more resistant to cold than to heat. Although some are killed by freezing, most bacterial spores and a large number of organisms survive and get revitalised when the food is thawed. 

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