Summary

12.1  The Language of Epidemiologists

• Epidemiology is the science underlying public health.
• Morbidity means being in a state of illness, whereas mortality refers to death; both morbidity rates and mortality rates are of interest to epidemiologists.
• Incidence is the number of new cases (morbidity or mortality), usually expressed as a proportion, during a specified time period; prevalence is the total number affected in the population, again usually expressed as a proportion.
• Sporadic diseases only occur rarely and largely without a geographic focus. Endemic diseases occur at a constant (and often low) level within a population. Epidemic diseases and pandemic diseases occur when an outbreak occurs on a significantly larger than expected level, either locally or globally, respectively.
• In the United States, the Centers for Disease Control and Prevention monitors notifiable diseases and publishes weekly updates in the Morbidity and Mortality Weekly Report.

12.2  Tracking Infectious Diseases

• Early pioneers of epidemiology such as John Snow, Florence Nightingale, and Joseph Lister, studied disease at the population level and used data to disrupt disease transmission.
• Descriptive epidemiology studies rely on case analysis and patient histories to gain information about outbreaks, frequently while they are still occurring.
• Retrospective epidemiology studies use historical data to identify associations with the disease state of present cases. Prospective epidemiology studies gather data and follow cases to find associations with future disease states.
• Analytical epidemiology studies are observational studies that are carefully designed to compare groups and uncover associations between environmental or genetic factors and disease.
• Experimental epidemiology studies generate strong evidence of causation in disease or treatment by manipulating subjects and comparing them with control subjects.

12.3  Modes of Disease Transmission

• Reservoirs of human disease can include the human and animal populations, soil, water, and inanimate objects or materials.
• Contact transmission can be direct or indirect through physical contact with either an infected host (direct) or contact with a fomite that an infected host has made contact with previously (indirect).
• Vector transmission occurs when a living organism carries an infectious agent on its body (mechanical) or as an infection host itself (biological), to a new host.
• Vehicle transmission occurs when a substance, such as soil, water, or air, carries an infectious agent to a new host.
• Healthcare-associated infections (HAI), or nosocomial infections, are acquired in a clinical setting. Transmission is facilitated by medical interventions and the high concentration of susceptible, immunocompromised individuals in clinical settings.