Principles of Disease and Epidemiology

• Pathology is the scientific study of disease.
• Pahotology came from the two Latin words: pathos = suffering and logos = science.
• Three concerns of pathology:

  1. Etiology – the cause of the disease
  2. Pathogenesis – the manner in which a disease develops
  3. The structural and functional changes brought about by disease and their effects on the body
• Infection - the invasion or colonization of the body by pathogenic microorganisms
• Disease - occurs when an infection results in any change from a state of health. Examples are AIDS, UTI, Athlete's foot, Pneumonia.
• Normal characteristic microbial populations begin to establish themselves in utero.
• The placental microbiome consists of only few different bacteria, mostly Enterobacteriaceae and Propionibacterium, found in the newborn’s intestine..
• A typical human body contains 3 x 10^13 body cells, and harbors as many bacteria cells—an estimated 4 x 10^13 bacterial cells.
• Began in 2007 to analyze microbial communities called microbiomes that live in and on the human body.
• The body's normal microbiota (or normal flora) is composed of microorganisms that establish more or less permanent residence but do not produce disease under normal conditions.
• The transient microbiota is composed of microbes that may be present for several days, weeks, or months and then disappear.\
• Many factors determine the distribution and composition of the normal microbiota including:

  1. Nutrients

  2. Physical and chemical factors

  3. The host’s defenses

  4. Mechanical factors
• Factors affecting microbial growth and normal microbiota composition: Temperature, pH, Available oxygen and carbon dioxide, salinity, sunlight
• Germ-free animals have undeveloped immune systems and are unusually susceptible to infection and serious disease.
• Microbial Antagonism - Also called competitive exclusion
• The normal microbiota protect the host against colonization by potentially pathogenic microbes by:

  1. competing for nutrients.

  2. producing substances harmful to the invading microbes.

  3. affecting conditions such as pH and available oxygen.
• The normal microbiota of the large intestine effectively inhibit Clostridium difficile, possibly by making host receptors unavailable, competing for available nutrients, or producing bacteriocins.
• E. coli cells produce bacteriocins, proteins that inhibit the growth of other bacteria of the same or closely related species, such as pathogenic Salmonella and Shigella.
• Commensalism - one organism benefits and the other is unaffected
• Mutualism - benefits both organisms  Bacteria in the large intestine
• Parasitism - one organism benefits by deriving nutrients at the expense of the other.
• Opportunistic pathogens are microbes that do not cause disease in their normal habitat in a healthy person but may do so in a different environment..
• Some diseases such as polio, Lyme disease, and tuberculosis have a well- known etiology.
• Other diseases have unknown etiology (e.g. Alzheimer’s disease).
• Robert Koch- discovered that Bacillus anthracis produced spore
• Symptoms are changes in body function, such as pain and malaise, that are subjective and not apparent to an observer.
• Signs are objective changes the physician can observe and measure (e.g. lesions, edema, fever, and paralysis).
• A syndrome is a specific group of symptoms or signs that may always accompany a particular disease.
• The diagnosis of a disease is made by evaluation of the signs and symptoms, together with the results of laboratory tests.
• Communicable diseases are diseases in which an infected person transmits an infectious agent, either directly or indirectly, to another person who, in turn, becomes infected (e.g. chickenpox, measles, influenza, genital herpes, typhoid fever, and tuberculosis.
• Contagious diseases are diseases that are very communicable and capable of spreading easily and rapidly from one person to another.
• Noncommunicable diseases are not spread from one host to another (e.g. tetanus caused by Clostridium tetani).
• Incidence – the number of people in a population who develop a disease during a particular time period.
• Prevalence - the number of people in a population who develop a disease at a specified time, regardless of when it first appeared.
• Frequency of Occurrence

  1. Sporadic disease – the disease occurs only occasionally (e.g. typhoid fever in USA).
  2. Endemic disease – a disease constantly present in a population (e.g. common cold).
  3. Epidemic disease – a certain disease is acquired by many people in a given area in a relatively short period (e.g. influenza, AIDS, gonorrhea).
  4. 4. Pandemic disease – occurs worldwide (e.g. COVID-19)
• Severity or Duration of a Disease:

  1. Acute disease – develops rapidly but lasts only a short time (e.g. influenza).
  2. Chronic disease – develops more slowly; body’s reactions may be less severe; likely to continue or recur for long periods (e.g. infectious mononucleosis, tuberculosis, and hepatitis B)
  3. Subacute disease – intermediate between acute and chronic diseases (e.g. subacute sclerosing panencephalitis).
  4. Latent disease – the causative agent remains inactive for a time but then becomes active to produce symptoms of the disease (e.g. Varicellovirus and Simplexvirus).
• Herd immunity – a state wherein most people in a population are immune to a particular disease, limiting outbreaks to sporadic cases.
• Extent on which the host’s body is affected:

  1. Local infection – the invading microorganisms are limited to a relatively small area of the body (e.g. boils and abscesses).

  2. Systemic or generalized infection – microbes or their products are spread throughout the body by the blood or lymph (e.g. measles).

  3. Focal infection – arises from infections in areas such as the teeth, tonsils, or sinuses
• State of Host Resistance:

  1. Primary infection – an acute infection that causes the initial illness (e.g. HIV infection and boils).

  2. Secondary infection – caused by an opportunistic pathogen after the primary infection has weakened the body’s defenses (Pneumocystis pneumonia, streptococcal bronchopneumonia).

  3. Subclinical (or inapparent) infection – does not cause any noticeable illness (e.g. Poliovirus and hepatitis A virus).
• A predisposing factor is one that makes the body more susceptible to disease or alters the course of a disease.