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SAI International School
                                            Subject- Biology, Class-VIII
                                    Topic- Microorganisms: Friend and Foe
                                       Subtopic- Harmful Microorganisms
                                                       Module- 14

                                                     Lesson Notes

               HARMFUL  MICROORGANISMS:  Some  microorganisms  cause  diseases  in  plants,
               animals  and  humans.  The  disease-causing  microorganisms  are  called  pathogens.
               Some microorganisms spoil food and damage clothes as well as leather.


               The diseases caused by microorganisms are transmitted from a diseased person to a
               healthy  person  and  are  called  communicable  diseases.  The  pathogens  of
               communicable  diseases  are  transmitted  through  air,  water,  direct  contact,  food  and
               carriers etc.

               BY  AIR:  When  a  diseased  person  sneezes,  thousands  of  microorganisms  come  out
               with  droplets  of  moisture.These  microorganisms  spread  in  air  and  enter  into  other
               persons  in  surrounding  through  inhaled  air.Examples  of  such  diseases  are,  chicken
               pox,tuberculosis  and  common  cold  etc.  One  should  cover  his  or  her  mouth  with
               handkerchief while sneezing and coughing.

               BY  WATER:  Pathogens  of  diseases,  like  cholera,hepatitis  and  dysentery,  etc.  are
               transmitted through contaminated water.

               BY  CARRIER:  The  living  organisms,  like  mosquitoes,flies  and  other  animals  which
               spread diseases are called carriers.

               Flies sit on garbage and excreta, pathogens stick to their bodies and when they sit on
               uncovered  food,  the  food  gets  contaminated.  By  eating  such  contaminated  food,  the
               person gets infected and becomes sick. We should not eat uncovered food to prevent
               such  diseases.  Example-  Cholera.  Female  Anopheles  mosquitoes  are  the  carriers  of
               malaria  parasites  and  female  Aedes  mosquitoes  are  the  carriers  of  dengue  virus.
               Mosquitoes breed in water. So to prevent such diseases, we should not allow water to
               get  collected  anywhere,  such  as,  tyres,  coolers,  flower  pots,  etc.We  should  keep  the
               surroundings  clean  and  dry.  The  causative  organisms  of  different  diseases  and
               prevention methods are as below:


               Human disease            Causative organism  Mode of                    Preventive
                                                               transmission            measures
               Tuberculosis             Bacteria               Air                     Patient should be
               Measles                  Virus                  Air                     kept in complete
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