(i) Organisms that cannot be seen with eyes alone or with naked eyes are called microbes.
(ii) Microbes are useful to humans in many ways:
They help in the production of curd, cheese, in cleaning the environment etc.
(iii) Diseases caused by microbes are polio, malaria etc.
(iv) Sneha is attentive, responsive and curious.

The body produce antibodies when a disease carrying microbe enters in it. The body also retains memory of this encounter to fight the microbe if it enters again. A vaccine consists of dead or weakened microbes which when introduced in a healthy body, produce suitable antibodies to fight and kill themselves. These antibodies remain in the body to protect it from disease causing microbes thus develop immunity.

The bacteria are classified into three types on the basis of their shape:
(a) Rod shaped (Bacillus) :
Example: Lactobacillus

(b) Round shaped (Coccus) :
Example: Streptococcus

(c) Spiral shaped
Example: Treponema

Virus:

Amoeba:

Salt and edible oils are the common chemicals generally used to check the growth of microorganism. Sodium benzoate and metabisulphate are common preservatives.

The milk is heated to 70°C for 15 seconds and then suddenly chilled and stored. By doing so, it prevents the growth of microbes. This process was discovered by Louis Pasteur. It is called Pasteurization.

Beneficial effects of bacteria are as follows :
(i) They give us various milk products like curd and cheese
(ii) They cause immune enhancement and prevents intestinal disorders
(iii) They decompose organic matter to give nutrients to plants.

Yeast plays an important role in baking industry. It reproduces rapidly and produces CO₂ during respiration. Bubbles of the gas fill the dough and increase its volume, making the products such as bread, cake and fluffy.

The 'unused kneaded flour' if left in warm conditions, gets infected by microbes which cause fermentation and spoils the flour. The pooris would remains in relatively good condition because these were deep fried in heated oil that kills microbes.

Microorganisms survive under adverse conditions by forming a hard and protective covering called cyst around them. On return of favourable conditions, the cyst wall breaks and microorganisms resume their life cycles.

Microorganisms are harmful for human beings in many ways. They cause diseases in humans, plants and animals, e.g.
(i) In humans, bacteria cause diseases such as cholera, typhoid, tuberculosis, etc.
(ii) In cattle, the foot and mouth disease is caused by a virus.
(iii) Also several microbes cause diseases in plants, e.g. tikka disease, late blight of potato. Some microbes grow on food and secrete toxic substances which make the food poisonous.

Microorganisms are useful for the humans and plants in following ways:
(i) They are used in preparation of food items such as breads.
(ii) Bacteria are used in the conversion of milk into curd.
(iii) They are used for the large scale production of alcohol, wine, and acetic acid.
(iv) A number of medicines such as antibiotics are being produced from bacteria and fungi.
(v) They are used to check microbial infections in animals.
(vi) Vaccines are made by dead or weakened microbes.
(vii) Some bacteria and blue-green algae increase the soil fertility by fixing atmospheric nitrogen.
(viii) They decompose the dead organic wastes of plants and animals.
(ix) They are used in cleaning up of the environment.
(x) Certain microbes are also used in the biological treatment of sewage and industrial effluents.

Microorganisms are divided into four major groups which are as follows:
(i) Bacteria These are single-celled organisms which are harmful, as they cause disease and some of them may be useful as in. vaccines.
(ii) Fungi These are non-green plants which consume their food from the dead organic matter.
(iii) Protozoa These are unicellular animals which causes diseases like malaria and dysentery.
(iv) Algae These are photoautotrophic organisms, that may be unicellular or multicellular. They play important role in medicines, agriculture, industry and as food. Some microbiologists also classify viruses as microorganisms, but other consider these as non-living. They are those microbes which reproduce only inside the cells of host organisms. They cause diseases like cold, polio, AIDS, etc.