Principles of Disease and Epidemiology Notes

Pathology and disease vocabulary

• Pathology: the study of disease.
• Etymology and roots:
  • Pathos = to suffer; prefixes related to disease include patho- (pathogen, pathology, pathogenesis).
  • Etiology: the cause of the disease (e.g., virus, bacterium, chemical, trauma, mutation).
  • Pathogenesis: how the disease develops; genesis means to create, so it’s about the physiological processes that create the disease state.
• Disease vs infection:
  • A disease is an abnormal state of the body; not in homeostasis and not functioning normally.
  • Pathogens invading and colonizing the body define infection; exposure to a toxin without pathogens is not an infection.
• Signs vs symptoms:
  • Symptoms: things felt by the patient (e.g., fatigue, pain, headache) and are not directly measurable.
  • Signs: observable or measurable indicators (e.g., body temperature, blood pressure, rash).
• Syndrome vs disease:
  • Syndrome = a group of signs and symptoms without a clear etiology.
  • Disease = a condition where the cause and signs/symptoms are linked.
• Quick recap of key terms:
  • Pathology: study of disease.
  • Etiology: cause of disease.
  • Pathogenesis: development of disease.
  • Disease vs infection: abnormal state vs presence of pathogens.
  • Signs vs symptoms: observable vs felt indicators.
  • Syndrome vs disease: complex presentation without a single clear cause vs a defined causal link.

Classifications of infectious disease

• Communicable: capable of spreading from one host to another.
• Contagious: a higher level of communicability; spreads rapidly and easily.
  • Examples and distinctions:
  • Measles: highly contagious.
  • HIV/AIDS: communicable, but not as easily transmitted as measles.
• Noncommunicable: not spread between hosts.
  • Examples: congenital/genetic diseases such as cystic fibrosis.

The development and spread of disease: key concepts

• Incubation period:
  • Time from initial infection to first signs or symptoms.
  • For a virus: from infection to the appearance of symptoms.
• Prodromal period:
  • Shortly after incubation; early, nonspecific symptoms (e.g., feeling unwell, mild throat irritation).
• Period of illness:
  • Disease is most severe; characteristic signs and symptoms are present; high microbial activity or toxin effects drive pathology.
  • The severity often relates to the number of microbes or the amount of toxin produced; specifics depend on the pathogen.
• Period of decline:
  • Signs and symptoms wane as the immune system controls the infection or as treatment reduces pathogen burden.
  • Example: influenza infection waning as immunity and/or antivirals reduce viral load.
• Convalescence:
  • Return to pre-disease state; immune system down-regulates the heightened response.
  • Antibodies may circulate for a period after recovery.
  • Convalescent serum (antibodies from recovered patients) has been discussed for various diseases, including COVID-19.
  • Example note: NIH clinical trials in 2020 explored convalescent plasma for COVID-19; as of 2020-09-22, data were not yet conclusive and trials were expanding.
  • Important nuance: Some interventions that seem obviously beneficial may not prove effective; rigorous testing is essential.

Disease progression terms and their meanings

• Acute disease:
  • Rapid onset with signs/symptoms that develop quickly; period of illness is relatively short.
  • Example: influenza; onset within $1 ext{ to } 4 ext{ days}$ of infection; duration typically $1 ext{ to } 2 ext{ weeks}$ (occasionally up to 3 weeks).
• Chronic disease:
  • Slow onset of symptoms; long duration, potentially lasting for years or decades.
  • Example: hepatitis C; signs may appear after weeks and persist for decades.