The dead organic matter disappears after sometime because the microorganisms decompose dead organic waste of plants and animals into simple substances. They are again used by other plants and animals.

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 that comes in packets does not get spoiled because it is pasteurised. Pasteurised milk can be consumed without boiling as it is free from harmful microbes. The milk was heated to about 70⁰ C for 15-30 second and then cooled suddenly and finally packed. By doing so, the growth of microbes was prevented. This process is called pasteurisation.

The reason could be that the chaat was contaminated by pathogenic microorganisms. It can be due to unhygienic condition near the shop or the utensil used for serving could have been contaminated.

Antibiotics are tablets, capsules or injections which are used to kill or stop the growth of pathogens, i.e. disease causing microbes. Penicillin, tetramycin are the examples of antibiotics.
Precautions to be taken are
(i) It should be taken in proper doze only on the advice of a qualified doctor.
(ii) Complete course prescribed by the doctor should be taken.
(iii) It should not be taken unnecessarily as it may kill useful bacteria in the body.

By introducing vaccine in body, the body fights and kills the bacteria by producing suitable antibodies. The antibodies remain in the body and the child is protected from the disease causing microbes.

Yeast reproduces rapidly and produces carbon dioxide during respiration. Bubbles of gas fill the dough and increase its volume. This is the basis of the use of yeast in baking industry.

No, antibiotics should not be used in cold and flu because these diseases are caused by viruses. And antibiotics are not effective against viruses.

Chickenpox and ringworm are diseases that can spread through direct contact with infected person.

Amoeba is a protozoan in the list given.

Uses of Bacteria:
(i) They are used to increase soil fertility by fixing nitrogen.
(ii) Some bacteria are used to produce antibiotics.
(iii) Lactobacillus bacterium converts milk into curd.
(iv) Some bacteria help in many functions of our body.
Uses of Fungi:
(i) Yeasts are used to prepare alcohol and vinegar by fermentation.
(ii) Mushrooms are eaten as food.
(iii) Penicillin is an antibiotic formed by a fungus called Penicillium.
Uses of Algae:
(i) Algae are used to make jellies.
(ii) Chlorella is used to obtain proteins.
(iii) Silicafrom diatoms are used in toothpastes.

Micro-organisms spoil our food. The process of keeping our food safe from micro-organisms is called food preservation. Various methods of food preservation are :
(i) Drying: It reduces the moisture of food material which prevents growth of micro-organisms. 
(ii) By common salt and sugar: Meat and fish are covered with dry salt to check the growth of bacteria. Sugar reduces the moisture content, which inhibits the growth of bacteria that spoil food.
(iii) By oil and vinegar: Use of oil and vinegar prevents spoilage of pickles because bacteria cannot live in such an environment.
(iv) Refrigeration and freezing: Low temperature retards the growth of microbes. Thus, it is safe for food.
(v) By chemicals: Chemicals such as sodium benzoate and potassium metabisulphite are used in jams, squash and ketchup.

Micro-organisms are" also very harmful to us. They cause a number of diseases in humans as well as in other animals. Common cold, Tuberculosis, Measles, chicken pox, polio, cholera, typhoid, hepatitis-B. malaria are some common human diseases caused by micro-organisms. Anthrax is a serious disease in animals caused by microbes. They also make food items unfit for use by food poisoning. They also spoil clothes and leather products. They also cause diseases in plants like blights in potatoes, sugarcane, oranges, etc. They also reduce the crop production.