2.  Few microorganisms are pathogenic (causing disease).  Some microorganisms are beneficial to the host.  Microorganism that have more or less permanent residence (colonize) but do not produce disease are part of body’s normal microbiota or normal flora.  Others called transient microbiota may be present for several days, weeks or months and then disappear.

3.  The same pathogen must be present in every case of the disease.  The pathogen must be isolated from the diseased host and grown in pure culture.  The pathogen isolated from the pure culture must cause the same disease when it is inoculated into a healthy, susceptible laboratory animal.  The pathogen must be isolated from the inoculated animal and must be shown to be the original organism.

4.  Koch’s postulates are modified to establish etiologies of diseases caused by viruses and some bacteria, which cannot be grown on artificial media.  Some diseases such as tetanus have unequivocal signs and symptoms.  Some diseases such as pneumonia and nephritis may be caused by a variety of microbes.  Some pathogens like Streptococcus pyogenes cause several different diseases.  Certain pathogens such as HIV cause disease in humans only that raises ethical questions.

5.  There must be a reservoir of infection as a source of pathogens for an infectious disease to occur.  The pathogen must be transmitted to a susceptible host by direct contact, indirect contact or by vectors.  Transmission is followed by invasion in which the microorganisms enters the host and multiplies.  Then microorganism injures the host through a process called pathogenesis.  Extent of injury depends on degree to which host cells are damaged.  Occurrence of disease finally depends on resistance of host to the activities of the pathogen.

6. A predisposing factor is one that makes the body more susceptible to a disease or may alter the course of the disease. Examples include -  Gender : females have more urinary tract infections than males  Genetic background: e.g sickle cell anemia occurs when genes for the disease is inherited from both the parents  Climate and weather: In temperate regions respiratory diseases occurs more during winter.  Inadequate nutrition, Fatigue, Age  Habits, lifestyle, occupation  Preexisting illness, chemotherapy  Emotional disturbances

7.  A continual source of infection (that provides a pathogen with adequate condition for survival and multiplication and an opportunity for transmission) is called a reservoir of infection.  It can be human, animal or nonliving.  People who have a disease or are carriers of pathogenic microorganisms are human reservoirs of infection. E.g. carriers of AIDS, hepatitis  Diseases that occur in wild and domestic animals and can be transmitted to humans are called zoonoses. E.g. rabies found in dogs, cats, foxes.  Nonliving reservoirs of infectious disease are soil and water. E.g. fungi present in soil causes ringworm, Vibrio cholerae present in water causes cholera and Salmonella typhi in water cause typhoid.

8.  There are three principal routes of transmission of disease - contact - vehicles - vectors  Contact transmission is the spread of an agent of disease by direct contact, indirect contact, droplet transmission.  Transmission by direct contact involves close physical contact between the source of the disease and a susceptible host (person-to-person transmission) e.g. respiratory tract diseases, sexually transmitted diseases.  Indirect contact transmission occurs when the agent of disease is transmitted from its reservoir to a susceptible host by means of a nonliving object known as fomite. e.g of fomites are tissues, towels.

10.  Transmission via saliva or mucus in coughing or sneezing is called droplet transmission.  Transmission by a medium such as water, food or air is called vehicle transmission.  In waterborne transmission, pathogens are usually spread by water contaminated with untreated or poorly treated sewage.e.g cholera disease  In foodborne transmssion, pathogens are generally transmitted in foods that are incompletely cooked, poorly refrigerated or prepared under unsanitary conditions. e.g. Food poisoining.  Airborne transmission refers to pathogens carried on water droplets or dust for a distance greater than 1 meter.

12.  Animals that carry pathogens from one host to another are called arthropod vectors.  Arthropod vectors transmit disease by 2 methods : mechanical transmission biological transmission  Mechanical transmission is the passive transport of the pathogens on the insect’s feet or other body parts.  Biological transmission is an active process in which the arthropod bites an infected person or animal and ingests some of the infected blood.

13.  Emerging infectious diseases are new diseases with diseases with increasing incidence.  Epidemiology is the science that studies when and where diseases occur and how they are transmitted in populations.  Bacteremia : refers to presence of bacteria in the blood.  Sepsis: If the bacteria multiply in the blood the condition is called sepsis.  Toxemia: refers to presence of toxins in the blood.  Viremia : refers to presence of virus in the blood.

14.  An infection that develops during the course of a hospital stay and was not present at the time the patent was admitted.  It results from the interaction of several factors: 1. Microbes in the hospital environment 2. The compromised (weakened) state of the host 3. The chain of transmission in the hospital

15.  Although attempts are taken to kill the growth of microbes in the hospital, the hospital environment is a major reservoir for a variety of pathogens.  One reason is that certain normal microbiota of human body are oppportunistic and present a particularly strong danger to hospital patients.  In fact, most of the microbes that cause nosocomial infections do not cause disease in healthy people but are pathogenic only for those whose defenses have been waekened.

16.  Particularly E.coli, Staphylococcuus aureus, Pseudomonas aeruginosa, Candida albicans.  One of the most important is Gram +ve Staph. aureus- mostly associated with blood (septicemia), surgical (wound), lower respiratory tract infetions.  Another is Pseudomonas aeruginosa – associated with lower respiratory tract and urinary tract and important cause of infection in surgical and burn patients.

17.  A compromised host is one whose resistance to infection is impaired by disease, therapy and burns.  Two principal conditions can compromise the host: 1. Broken skin or mucus membrane 2. A suppressed immune system

18.  Nosocomal infections spread rapidly easily and directly (between staff members and paients) and indirectly (between patients) for several reasons: 1. The crowding of patients in rooms and wards increases the chance of cross-infection 2. Hospital personnel moves from patient to patient 3. Many diagnostic and therapeutic hospital procedures provide a fomite route of transmission.

20.  Control measures aimed at preventing nosocomial infections vary from one institution to another, but certain procedures are generally implemented. It is important to reduce the number of pathogens to which patients arc exposed by using aseptic techniques, handling contaminated materials carefully, insisting on frequent and thorough hand-washing, educating staff members about basic infection control measures, and using isolation rooms and wards.

21.  In addition to hand-washing, tubs used to bathe patients should be disinfected between uses so that bacteria from the previous patient will not contaminate the next one.  Respirators and humidifiers provide both a suitable growth environment for some bacteria and a method of airborne transmission.  These sources of nosocomial infections must be kept scrupulously clean and disinfected, and materials used for bandages and intubation (insertion of tubes into organs, such as the trachea ) should be single-use disposable or sterilized before use.  Packaging used to maintain sterility should be removed aseptically.  Physicians can help improve patients' resistance to infection by prescribing antibiotics only when necessary, avoiding invasive procedures if possible, and minimizing the use of immunosuppressive drugs.

22.  EIDs are ones that are new or changing, showing an increase in incidence in the recent past, or a potential to increase in the near future.  An emerging disease can be caused by a virus, a bacterium, a fungus, a protozoan, or a helminth.  Several criteria are used for identifying an EID.  For example, some diseases present symptoms that are clearly distinctive from all other diseases. Some are recognized because improved diagnostic techniques allow the identification of a new pathogen.  Others are identified when a local disease becomes widespread, a rare disease becomes common, a mild disease becomes more severe, or an increase in life span permits a slow disease to develop.

23.  A variety of factors contribute to the emergence of new infectious  diseases: New strains, such as E. coli 0157:1-17 and avian influenza (H5N1), may result from genetic recombination between strains.  A new serovar, such as Vibrio cholarae 0139, may result from changes in or the evolution of existing microorganisms.  The widespread, and sometimes unwarranted, use of antibiotics and pesticides encourages the growth of more resistant populations of microbes and the insects (mosquitoes and lice) and ticks that carry them.  Global warming and changes in weather patterns may increase the distribution and survival of reservoirs and vectors, resulting in the introduction and dissemination of diseases such as malaria and Hantavirus pulmonary syndrome.