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Viruses, Bacteria, Fungi, Protozoa and certain worms are the main organisms causing
diseases.
To cause disease, they must first gain entry into the body. Such entry must be either through
the skin or through the nose in the respiratory system or through the mouth to the alimentary
canal.
Some organism passes directly through the surface of the skin. Such is the case with the
spores of the fungus which causes ring worm. Bacteria frequently enter the skin through a
wound, causing inflammation of the wound.
The method of carrying this disease organism to the body is varied. The carrier of disease
organisms is called vectors. They are said to transmit diseases.
Many microorganisms enter through the nose or mouth and penetrate the delicate
membranes of the respiration system. Virus causing cold and influenza enter this way.
Parasitic bacteria, Protozoan, Viruses, etc., cause various communicable diseases in man.
Communicable disease is pathogenic diseases which spread from person to person either
directly or indirectly. The following table shows some of the common communicable
diseases in man.
Bacteria:
Curd contains lacto bacillus bacteria which help to change the milk into curd. Bacteria are
unicellular and microscopic belonging to the world of microbes. The study of bacteria is
called Bacteriology. Bacteria were first observed under a microscope by the Dutch scientist
Anton Von Leeuwenhoek in 1675. Later Louis Pasteur, Robert Koch and Lord Lister carried
out detailed studies on bacteria.
The structure of bacteria can be studies with the help of an electron microscope. The bacteria
cell is a prokaryotic cell. It has a rigid cell wall protecting the cell and giving a definite shape
to it. The living material inner the cell wall is called protoplasm. It is differentiated into cell
membrane, nuclear material and cytoplasm. Membrane bound organelles like Golgi bodies,
mitochondria, endoplasmic, reticulum, lysosomes are absent. It contain bacteriochlorophyll
pigments. The nuclear material of a bacterial cell is made of a circular, DNA molecule. It is
not bound by nuclear membrane . There are thread like appendages which are called flagella,
the organs of motility. Pilli are minute, straight, hair like appendages and are considered to
be organs of attachment.
Bacteria are measured in microns. 1 microns = 1/1000 millimeter.