INFECTIOUS DISESSASE (Defination):
Any disease caused by invasion by a pathogen which subsequently grows and multiplies in the body.
a disease transmitted only by a specific kind of contact

Concept:
The history of the human species, it has been said, is the history of infectious disease. Over the centuries, humans have been exposed to a vast amount and array of contagious conditions, including the Black Death and other forms of plague, typhoid fever, cholera, malaria, influenza, and the acquired immunodeficiency syndrome, or AIDS. Only in the past few hundred years have scientists begun to have any sort of accurate idea concerning the origin of such diseases, through the action of microorganisms and other parasites. Such understanding has led to the development of vaccines and methods of inoculation, yet even before they made these great strides in medicine, humans had an unseen protector: their own immune systems.

Infection and Humanity:There are two basic types of disease: ones that are infectious, or extrinsic, meaning that they are contagious or communicable and can be spread by contact between people, and ones that are intrinsic, or not infectious.a person can get an infection, such as tetanus or salmonella, without necessarily having a disease that can be passed on through contact with others in the same way that colds, malaria, or syphilis is spread.

Immune Machanism:
The human body has numerous mechanisms for protecting itself from infectious disease, the first line of defense being the skin. Skin shields us all the time from unseen attackers and generally is able to prevent pathogens from entering the body; however, any break in the skin, such as a cut or scrape, provides an opening for microorganisms to invade the body. Germs that normally would be prevented from entering the body are able to invade the bloodstream through such openings. This is why it is so very important, in any situation involving potential contact with infection, to protect the skin.

Transmission of Disease:
Infectious diseases are transmitted easily from one person to another.Pathogens can be spread by many methods other than direct contact, including through water, food, air, and bodily fluids—blood, semen, saliva, and so on. For instance, any time a person with an infection coughs or sneezes, they may be transmitting illness. This is how diseases such as measles and tuberculosis are passed from person to person. AIDS and various STDs, as well as many other conditions, such as hepatitis, are transferred when one person comes into contact with the bodily fluids of another. This is the case not only with sexual intercourse but also with blood transfusions and any number of other interactions, including possibly drinking after someone.Additionally, diseases may be transferred by vectors—animals (usually insects) that carry microorganisms from one person to another.

Diagnosis and Therapy:
Diagnosis is initially by medical history and physical examination, and imaging (such as X-rays), but the principal tool in infectious disease is the microbiological culture. In a culture, a growth medium is provided for a particular agent. After inoculation of a specimen of diseased fluid or tissue onto the medium, it is determined whether bacterial growth occurs.
When a culture has proven to be positive, the sensitivity (or, conversely, the antibiotic resistance) of an agent can be determined by exposing it to test doses of antibiotic. This way, the microbiologist determines how sensitive the target bacterium is to a certain antibiotic.

The work of an Infectiologist:
Doctors who specialise in the medical treatment of infectious disease are called infectiologists or infectious disease specialists. Generally, infections are initially diagnosed by primary care physicians or internal medicine specialists. For example, an "uncomplicated" pneumonia will generally be treated by the internist or the pulmonologist (lung physician).

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