Principles of Disease and Epidemiology

General Principles Related to Disease  Pathology is the scientific study of disease and it involves three things;  The study of the cause or etiology of a disease.  The study of the manner in which the disease develops  The study of the structural and functional changes brought about by the disease and its final effects on the host

General Principles Related to Disease  Infection refers to the invasion or colonization of the body by pathogenic microorganisms.  Disease refers to any change from a normal state of health or an abnormal state in which part or all of the body is not properly adjusted or is not capable of carrying on its normal functions.

General Principles Related to Disease  Although infection and disease are often used interchangeably, infection does not necessarily indicate disease. For example:  Someone may be infected with HIV, but show no symptoms of the disease.  Normal flora may colonize or infect specific areas of the body without causing disease. They may, however, cause disease given the right circumstances.  E. coli is NF in the stool, but it is the number one cause of urinary tract infections.

General Principles Related to Disease  Viridans group Streptococcus is normal flora in the oropharyngeal area, but can cause subacute bacterial endocarditis when it gains entrance into the bloodstream  Both of the above organisms are considered to be opportunistic pathogens.  In addition to the usual NF, some people are colonized by microorganisms that are generally considered to be pathogenic, but which don’t actually cause disease in those individuals.  These people are called carriers and they can transmit the pathogenic microorganisms to other people who then develop the disease normally caused by that microorganism.

General Principles Related to Disease  Neisseria meningitidis  Salmonella typhi  Causes of disease – some diseases have a well understood etiology, others have a partially understood etiology, and others have an undetermined etiology. The main categories of disease include:  Infectious disease – caused by disease producing microorganisms

General Principles Related to Disease  Nutritional deficiency disease – caused by the lack of a particular, necessary nutrient  Congenital disease* – is present at birth and is the result of some condition that occurred in utero (maternal infection, use of drugs or alcohol, etc.)  Inherited disease* – are passed to the child via the parent’s reproductive cells  * birth defects may result from these

General Principles Related to Disease  Metabolic diseases – result from abnormalities in the biochemistry of body function.  Many are congenital or inherited disorders.  Degenerative diseases – this occurs when there is a wearing down of part of the body leading to loss of function.  This may be due to aging, excessive caloric intake, radiation, errors in gene function, etc.

General Principles Related to Disease  Neoplastic diseases – these are tumors which are new growth of cells or tissues.  Tumors may be benign or malignant. What is the difference?  Immunologic diseases – this occurs when some of our immunologic defenses attack our own bodies.  These diseases are also called autoimmune diseases.  Iatrogenic disease – are caused by health care personnel during the delivery of health care  Could be due to use of contaminated equipment  Could be caused by the administration of drugs

General Principles Related to Disease  Nosocomial disease – is a hospital acquired disease  Hospitals provide the perfect environment for microorganisms, in part because patients are in a weakened condition and therefore more susceptible to disease.  Up to 15% of hospitalized patients get a nosocomial infection.  Most nosocomial infections are caused by opportunistic pathogens that are typically considered to be NF.

Factors contributing to nosocomial infections

Types of nosocomial infections (Septicemia)

General Principles Related to Disease  Psychogenic diseases – are caused, at least in part, by emotional factors  Idiopathic diseases – diseases that have an undetermined cause  Koch’s postulates – in the 1800’s Robert Koch developed a series of steps, called Koch’s postulates, that provide a framework for the study of the etiology of an infectious disease:

General Principles Related to Disease  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 from pure culture must cause the same disease when inoculated into a healthy, susceptible host.  The pathogen must again be isolated from the inoculated animal and must be shown to be the same as the originally isolated organism.

Application of Koch’s postulates

General Principles Related to Disease  Why is not not possible to identify the etiology of all infectious diseases following Koch’s postulates?  Some pathogens can’t be isolated on artificial culture media. Examples?  Some pathogens cause more than one kind of disease. Examples?  Some diseases are cause by a variety of different pathogens. Examples?

General Principles Related to Disease  How do infections or diseases spread?:  Reservoirs – for a disease to perpetuate itself, there must be a continual source of the infection called a reservoir of infection. Reservoirs my be:  Humans – Carriers or sick individuals during the incubation, acute, or convalescent phases of a disease (depends upon the particular disease)  Animals – diseases found in animals are called zoonosis  Non-living entities such as soil or water  Transmission of disease – The causative agents of infection can be transmitted from the reservoir of infection to a susceptible host via four different routes:

General Principles Related to Disease  Contact  Direct contact –the infection is spread from person to person via touching, kissing, or sexual intercourse  Indirect contact – the disease is transmitted to a susceptible host by means of a non-living object called a fomite (contaminated needles)  Droplet transmission – agents of disease are spread by droplets contained in saliva and mucous that are discharged by sneezing, coughing, laughing, or talking.  The droplets travel only a short distance (< 1 meter) and for such short distances, the spread is not considered to be airborne.

Direct contact transmission

Droplet transmission

General Principles Related to Disease  Common vehicle transmission – refers to transmission of disease causing agents by a common inanimate reservoir to a large number of individuals  Food  Water  Blood  Drugs

Common vehicle transmission

General Principles Related to Disease  Airborne transmission – this refers to the spread of agents of infection by droplet nuclei or dust.  In this instance the particles travel > 1 meter from the reservoir to the host.  Vector transmission – a vector is an animal that carries the pathogen from one host to another.  Arthropods are the most common type of vector.  Arthropods can transmit disease by two general methods:

General Principles Related to Disease  Mechanical – the pathogen is transmitted on the insects feet or other body parts to the hosts food.  Biological – the arthropod bites an infected individual and acquires the pathogen which reproduces inside the insect.  The pathogen is then transmitted to a new host via the salivary glands or in the feces when the insect defecates.

Vector transmission

General Principles Related to Disease  Classification of infectious diseases – there are several different ways to classify diseases:  Based on how the disease is spread  Communicable disease – spreads from one host to another either directly or indirectly  Noncommunicable disease – doesn’t spread from host to host but is caused by an organism that  Normally inhabits the body and only occasionally causes disease or  Resides outside the body and produces disease when it is introduced into the body  Contagious disease – a disease that spreads easily from one person to another

General Principles Related to Disease  Diseases may be classified in terms of severity or duration  Acute disease – develops rapidly and lasts a relatively short time  Chronic disease – develops slowly, the body’s reactions are usually less severe, but the disease is continuous or recurrent for long periods of time  Subacute disease – is intermediate between acute and chronic  Latent disease – the causative agent remains inactive for a long period of time and then becomes active and produces symptoms

General Principles Related to Disease  Diseases can be classified based on the frequency of occurrence  Sporadic – occurs only occasionally  Endemic – constantly present in the population  Epidemic – many people in a given area acquire the disease in a short period of time  Pandemic – a world-wide epidemic

General Principles Related to Disease  Diseases may be classified based on the health of the body  Primary infection – an acute infection that causes the initial illness  Secondary infection – is caused by an opportunistic pathogen after the primary infection has weakened the body’s defenses  Inapparent or subclinical infection – doesn’t cause any notable illness (a problem in carriers)

General Principles Related to Disease  What are the signals of disease?  Symptoms – subjective changes in body function such as pain or malaise (not measurable)  Signs – objective changes that can be observed and measured such as fever, swelling, or a rash  Syndrome – a group of symptoms and signs that always accompany a particular disease

General Principles Related to Disease  What are the stages of disease?  Period of incubation – the time been acquiring the infection and the appearance of he first sins or symptoms.  This may be a constant time for every individual who acquires the infection or a variable time (depends upon the disease).  Prodromal period – when the first signs and symptoms appear.  Period of illness – when the disease is most acute and the overt signs and symptoms of the disease occur (an increase or decrease in WBCs may occur here).

General Principles Related to Disease  Period of decline – this is where the signs and symptoms subside.  If the decline occurs quickly, it is said to occur by crisis.  If the decline occurs over a longer period of time, it is said to occur by lysis.  Period of convalescsnce - this is where the person regains strength and the body returns to its pre-diseased state.

Stages of disease

General Principles Related to Disease  Epidemiology – is the study of when and where diseases occur and how they are transmitted in a population.  It is also concerned with various methods of controlling a disease such as use of drugs, vaccines, and control of human, animal and nonliving reservoirs.  Epidemiology is a major concern of state and federal public health departments.  The Centers for Disease Control and Prevention, CDC, in Atlanta, Georgia, is a central source of epidemiological information in the United States.