Pathology- It is the science of the nature and the causes of disease which involves changes in structure and functions.
Bacteriology- it is the science of micro organisms.

Infection and infectious diseases- The majority of human diseases are caused by biological agents i.e. living organisms, ranging in size from the smallest, filter passing, ultramicroscopic, parasitic, viruses, to large worms several feet in length of these. The microorganisms visible only under microscope are greater importance. The greatest of the recent advances in medicine have been the developments in chemotherapy in the last 40 years, with the resultant control over some but not all of the diseases. But as some diseases are controlled, others become more important and so on it goes.
All microorganisms do not produce disease. In fact, the body normally harbours several types of organisms which are considered ‘normal flora’. The majority are non-pathogenic i.e. cannot produce disease. Some are pathogenic but not at the numbers present, some organisms are even beneficial to us.

Some common terms used in Pathology
Tissue- is a collection of cells having similar structure and function
e.g. Muscle tissue, Nerve tissue
Organ- is a collection of various tissues having same origin and same function.  e.g. Liver, Lungs, Heart
System- is composed of various types of tissues subserving a definite function in the body.  e.g. Respiratory system, Digestive system Inflammation- means a characteristic local reaction of the living tissue in response to an injury or an infection. All inflammation will have the following features
a)     Redness (Rubor)
b)     Raised temperature (Calor)
c)     Swelling (Tumor)
d)     Pain at the site of inflammation (Dolor)
Histology- study of the structure of various tissues in health & disease is called histology.
Pathogenesis- study of the mechanism of development of a disease is called pathogenesis.
Pathogens- disease producing micro-organisms are called pathogens.
Commensals (non-pathogens) - all micro-organisms present within the body and in the environment are not harmful to human beings. The organisms which are normally present within the human body and are not harmful are called Commensals of non-pathogens. Many a time they subserve a beneficial purpose to human beings.
Degeneration- means withering away of cells or tissue.
Regeneration – means regrowth of cells of tissue.
Hyperplasia- means excessive proliferation (i.e. growth with the increase in the number of cells)
Hypoplasia- means sub-normal or less than normal rate of proliferation of cells
Aplasia- means total absence of proliferation of cells
Hypertrophy- means swollen u[ condition (i.e. increase in size) of cells/tissue/organ
Atrophy- means shrunken up condition (i.e. decrease in size) of cells/tissue/organ
Necrosis- means death of cells/tissue/organ
Gangrene- means massive necrosis (i.e. death of tissue over a wide area) due to lack of blood supply to the part concerned.

Classification of Micro organisms:
1. Bacteria- very minute living organisms belonging to the plant kingdom. They are 
   unicellular and contain no chlorophyll. They measure only a few microns in size.
   Bacteria are classified into three different manners. According to:
a) Shape  
    Spherical shaped    - Cocci
    Rod shaped           - Bacilli
    Spiral shaped         - Spirochetes

b) Grouping arrangement    
    Grapelike cluster     - Staphylococci
    Chainlike               - Streptococci

c) Staining method
    Red color              - Gram-ve    Meningococci, Gonococci
    Blue color              - Gram+ve   Staphylococci, Streptococci

2. Viruses- Ultramicroscopic organisms (smaller than bacteria). Unlike bacteria
   cannot be cultivated on artificial media. There are two types of viruses, small and
   large.
   Examples- Small- Chicken pox, small pox, measles, herpes, mumps
               Large- Atypical pneumonia lymphogranuloma venereum

3. Rickettsiae- Micro-organisms larger than viruses smaller than bacteria responsible
   for causing diseases like typhus, rocky mountain spotted fever (rare in India)

4. Protozoa- Minute unicellular organisms belonging to the animal kingdom   e.g. 
    E.Histolytica, G.Lamblia,

5. Fungi- Minute unicellular organism belonging from plant life (larger than bacteria)
    e.g. Candida, Actinomycetes