1. Infection and Infectious disease

2. Diseases are often classified in terms of how they behave within a host and within a given population. Classification of Infectious disese: Communicable Contagious Non-communicable

3. Communicable disease :Any disease that spreads from one host to another, either directly or indirectly,is said to be a communicable disease. Chickenpox, measles, genital herpes, typhoid fever, and tuberculosis are examples. Contagious diseases :Chickenpox and measles are also examples of contagious diseases, that is, diseases that are easily spread from one person to another

4. Noncommunicable disease: A noncommunicable disease is not spread from one host to another. These diseases are caused by microorganisms that normally inhabit the body and only occasionally produce disease or by microorganisms that reside outside the body and produce disease only when introduced into the body. An example is tetanus: Clostridium tetani produces disease only when it is introduced into the body via abrasions or wounds.

5. Occurrance of a Disease Incidence: The incidence of a disease is the number of people in a population who develop a disease during a particular time period. It is an indicator of the spread of the disease. Prevalence: The prevalence of a disease is the number of people in a population who develop a disease at a specified time, regardless of when it first appeared. Prevalence takes into account both old and new cases. It's an indicator of how seriously and how long a disease affects a population.

6. Depending on the occurance of the dieases, they are classified into 4 groups: 1. Sporadic disease: If a particular disease occurs only occasionally, it is called a sporadic disease; typhoid fever in the United States is such a disease. 2. Endemic disease: A disease constantly present in a population is called an endemic disease; an example of such a disease is the common cold.

7. 3. Epidemic disease:If many people in a given area acquire a certain disease in a relatively short period, it is called an epidemic disease; influenza is an example of a disease that often achieves epidemic status. 4.Pandemic disease: An epidemic disease that occurs worldwide is called a pandemic disease. We experience pandemics of influenza from time to time. Some authorities also consider AIDS to be pandemic. In 1918: Spanish flue pandemicThis flu outbreak orphaned children, closed down schools and businesses, and left the nation without important services because of ill workers. An estimated 675,000 Americans died in a matter of months, and 20 million people died across the globe before the pandemic subsided

8. Severity or duration of disease: Another useful way of defining the scope of a disease is in terms of its severity or duration. 1.Acute disease: An acute disease is one that develops rapidly but lasts only a short time; a good example is influenza. 2. Chronic disease: A chronic disease develops more slowly, and the body's reactions may be less severe, but the disease is likely to continue or recur for long periods. Infectious mononucleosis, and hepatitis B fall into this category.

9. 3.Subacute disease :A disease that is intermediate between acute and chronic is described as a subacute disease; an example is subacute sclerosing panencephalitis, a rare brain disease characterized by diminished intellectual function and loss of nervous function. 4. Latent disease: A latent disease is one in which the causative agent remains inactive for a time but then becomes active to produce symptoms of the disease; an example is Shingles, one of the diseases caused by varicella virus.

10. Extent of Host involvement: Local Infection: A local infection is one in which the invading microorganisms are limited to a relatively small area of the body. Some examples of local infections are boils and abscesses. Systemic Infection: In a systemic (generalized) infection, microorganisms or their products are spread throughout the body by the blood or lymph. Measles is an example of a systemic infection. Focal Infection: Very often, agents of a local infection enter a blood or lymphatic vessel and spread to other specific parts of the body, where they are confined to specific areas of the body. This condition is called a focal infection. Focal infections can arise from infections in areas such as the teeth, tonsils, or sinuses.

11. The presence of bacteria in the blood is known as bacteterimia. If the bacteria actually multiply in the blood, this condition is called sepsis Toxemia refers to the presence of toxin in the blood. Viremia refers to the presence of virus in the blood.

12. The state of host resistance also determines the extent of infections. A primary infection is an acute infection that causes the initial illness. A secondary infection is one caused by an opportunistic pathogen after the primary infection has weakened the body's defenses. Secondary infections of the skin and respiratory tract are common and are sometimes more dangerous than the primary infections. Pneomocystis pneumonia as a consequence of AIDS is an example of a secondary infection; streptococcal bronchopneumonia following influenza is an example of a secondary infection that is more serious than the primary infection. A subclinical (inapparent) infection is one that does not cause any noticeable illness. Poliovirus and Hepatitis A virus, for example, can be carried by people who never develop the illness.

13. Development of Diseases Once a microorganism overcomes the defenses of the host, development of the disease follows a certain sequence that tends to be similar whether the disease is acute or chronic. Incubation Period: The incubation period is the interval between the initial infection and the first appearance of any signs or symptoms. In some diseases, the incubation period is always the same; in others, it is quite variable. The time of incubation depends on the specific microorganism involved, its virulence (degree of pathogenicity), the number of infecting microorganisms, and the resistance of the host.

14. Prodormal Period: The prodromal period is a relatively short period that follows the period of incubation in some diseases. The prodromal period is characterized by early, mild symptoms of disease, such as general aches and malaise.

15. Period of Illness: During the period of illness, the disease is most severe. The person exhibits overt signs and symptoms of disease, such as fever, chills, muscle pain (myalgia), sensitivity to light (photophobia), sore throat (pharyngitis), lymph node enlargement (lymphadenopathy), and gastrointestinal disturbances. During the period of illness, the number of white blood cells may increase or decrease. Generally, the patient's immune response and other defense mechanisms overcome the pathogen, and the period of illness ends. When the disease is not successfully overcome (or successfully treated), the patient dies during this period.

16. Period of Decline: During the period of decline. the signs and symptoms subside. The fever decreases, and the feeling of malaise diminishes. During this phase, which may take from less than 24 hours to several days, the patient is vulnerable to secondary infections.

17. Period of Convalescence During the period of convalescence, the person regains strength and the body returns to its prediseased state. Recovery has occurred.

18. Development & stages of Disease