15.3 Epidemiology and Disease Control

Why population measures sit in Microbiology

/practice/nbce-part1Practice questions with detailed explanations

Quick Answer: Clinicians report reportable diseases to the state or local health department because state law says so. CDC compiles nationally notifiable conditions through NNDSS using CSTE case definitions; federal reporting is voluntary from jurisdictions. Isolation is for the sick; quarantine is for the exposed well. Sterilize critical instruments; disinfect tables. Incidence = new cases / population / time. Prevalence = existing cases / population. Emerging diseases are new or newly aggressive in a population.

Health agencies and the reporting ladder

Local and state health departments have the legal authority to require reporting. A physician, chiropractic physician where state statute includes DCs, laboratory, or hospital that identifies a listed condition must notify the jurisdiction on the published timeline (immediately for measles and meningococcal disease in many states; days for others). The list varies by state. Coccidioidomycosis is a teaching example: it is nationally notifiable, but some states where it is uncommon do not make it reportable locally, so CDC does not receive complete geographic coverage.

The Council of State and Territorial Epidemiologists (CSTE), working with CDC, votes position statements that define which conditions are nationally notifiable and how a case is classified (confirmed, probable, suspect). CDC’s National Notifiable Diseases Surveillance System (NNDSS) is the pipeline: jurisdictions send de-identified case data. Reporting from states to CDC is voluntary in the legal sense; it is a cooperative surveillance program, not a statute that puts every U.S. clinician on a direct phone tree to Atlanta. CDC analyzes the data, publishes weekly MMWR tables, and posts case definitions at the NNDSS / National Notifiable Diseases site (ndc.services.cdc.gov).

CDC (Atlanta) is the national public-health authority for surveillance, outbreak support, vaccine recommendations (often via ACIP), and infection-control guidelines. It does not license chiropractors and it does not replace the state board. WHO coordinates International Health Regulations (IHR 2005) and can declare a Public Health Emergency of International Concern (PHEIC); U.S. case reporting still starts locally. OSHA enforces workplace bloodborne-pathogen rules (sharps, HBV vaccination offers, exposure control plans). EPA registers many surface disinfectants. FDA regulates sterilants, liquid chemical sterilants, and vaccines as products. On a stem, match the agency to the job: OSHA for the clinic’s sharps container, EPA for a labeled table disinfectant, state epi for a measles report, CDC for the national case definition, WHO for a multi-country PHEIC.

2026 NNDSS facts worth knowing as current surveillance changes (CDC / CSTE implementation letters and the 2026 notice to NNDSS data users): COVID-19-associated pediatric mortality became a new nationally notifiable condition; Oropouche virus disease (non-congenital, and congenital disease in annual tables) is on the national list; dengue-like illness was removed while dengue and severe dengue remain notifiable; syphilis publication tightened toward confirmed/probable cases; conditions under standardized surveillance (not the weekly notifiable table) include flea-borne typhus and soil-transmitted helminth infections. Candida auris clinical infection has been nationally notifiable since 2019. Do not memorize an unpublished NBCE item count for this topic; do remember that the notifiable list changes by year and that the legal duty is state reportable first.

If you diagnose or strongly suspect a rapidly spreading vaccine-preventable disease (measles in a student with Koplik spots and a spinal-pain visit), report to the local health department and refer—do not wait for a federal website. That is identification and control, not a billing code.

The chain of infection and where you break it

Communicable disease requires a chain: infectious agent → reservoir → portal of exit → mode of transmission → portal of entry → susceptible host. Control methods named on the test plan are the cuts in that chain.

Modes of transmission (use the micro you already learned):

• Contact: direct (scabies, dermatophytes, C. auris skin) or indirect (fomites, Enterobius eggs).
• Droplet: large respiratory particles, typically <6 feet (influenza, pertussis, meningococcus).
• Airborne: droplet nuclei that remain aloft (measles, varicella, TB; Pneumocystis can spread airborne among susceptible hosts).
• Vehicle: water (Giardia, Cryptosporidium, cholera), food (Taenia cysticerci, Trichinella, Salmonella), blood.
• Vector: Anopheles (Plasmodium), Ixodes (Borrelia—bacteria), body louse (typhus).
• Vertical: transplacental Toxoplasma, congenital syphilis.

Isolation restricts people who are ill (or colonized, in some contact-precaution protocols) so their portal of exit does not reach new hosts. Standard precautions apply to all patients: hand hygiene, PPE by task, safe injection, respiratory hygiene. Transmission-based precautions add:

Quarantine restricts asymptomatic people who were exposed during the incubation window (classic: measles contacts who are nonimmune stay home through day 21). Isolation ≠ quarantine ≠ sterilization. A chiropractic office sending a febrile, coughing patient home is practicing isolation of the ill; asking unvaccinated siblings of a measles case to stay out of school is quarantine; autoclaving a needle is sterilization.

Vaccination removes susceptible hosts. Herd immunity threshold is approximately 1 − 1/R₀. Measles R₀ is often cited near 12–18, so coverage must be very high (~92–95%) to block sustained spread—those R₀ figures are epidemiologic teaching values, not NBCE-published constants. Live vaccines (MMR, varicella) generally must not go to pregnant or severely immunocompromised hosts; inactivated and recombinant vaccines are the alternative. Immunization types are taught in /study-guides/nbce-part1/microbiology-host-pathogens/immunology; here the population point is coverage and interruption of transmission.

Identification in outbreaks follows a CDC-style sequence you should be able to order: confirm the increase is real → verify diagnoses → write a case definition (person, place, time, clinical/lab criteria) → descriptive epidemiology → hypothesis → analytic study (cohort or case–control) → implement control as soon as you have a working hypothesis (do not wait for the p-value) → communicate. An outbreak (or epidemic, when larger) is more cases than expected in that population and time. Endemic means the baseline is constantly present (Valley fever in parts of Arizona). Sporadic means irregular single cases. Pandemic means sustained spread across regions or continents (COVID-19).

Sterilization, disinfection, antisepsis, and the Spaulding classes

These words are not interchangeable.

Spaulding classification (CDC infection-control teaching):

A chiropractic adjusting table that contacts intact skin is noncritical. Clean visible soil, then apply an EPA-registered low-level disinfectant with the labeled wet-contact time. If the table is contaminated with blood or with scale from weeping tinea, escalate cleaning and follow the bloodborne-pathogen plan. A needle or any device that breaks skin is critical and must be sterile and single-use or autoclaved in a validated cycle. Face paper is a barrier, not a sterilant.

Moist heat (steam autoclave): the classic teaching set is 121°C (250°F), 15 psi, ≥15 minutes (gravity displacement; wrapped loads often run longer; prevacuum cycles at ~132°C are shorter). Steam must contact all surfaces—air pockets fail the load. Biological indicators (Geobacillus stearothermophilus spores) monitor the sterilizer. Dry heat: higher temperature, longer time (e.g., 170°C for 60 minutes) for oils or powders that steam ruins. Ethylene oxide gas sterilizes heat-sensitive plastics but needs aeration because the gas is toxic. Hydrogen peroxide plasma is another low-temperature sterilant family. Filtration: HEPA removes 99.97% of 0.3 µm particles for air; membrane filters sterilize heat-labile liquids. UV-C disinfects surfaces and air in line of sight; it is not a substitute for an autoclave on a critical instrument.

Chemicals (know the gaps):

Alcohol hand rub is preferred for most routine hand hygiene when hands are not visibly soiled. Soap and water for visible soil, after restroom use, and after caring for spore or Cryptosporidium / norovirus situations where alcohol is weak. Gloves do not replace hand hygiene.

Population health measures: incidence, prevalence, and related rates

Incidence is the number of new cases of a disease in a defined population at risk during a defined time. Cumulative incidence (risk) = new cases / population at risk over the period. Incidence rate (density) = new cases / person-time. Prevalence is the number of existing cases (new and old still alive with disease) divided by the population, either at a point in time or over a period.

In a steady state with no migration and constant rates, prevalence ≈ incidence × average duration. The bar chart above is a teaching cartoon: two diseases with the same incidence of 10 new cases per 1,000 people per year. If mean duration is about 0.2 years (~10 weeks), point prevalence is about 2 per 1,000. If mean duration is 5 years (a chronic mycosis or untreated HIV before modern therapy), point prevalence is about 50 per 1,000. Short, explosive illnesses (influenza) can have incidence >> point prevalence. Long illnesses (HIV, TB infection, some parasitic diseases) can have prevalence >> incidence. That is why you cannot read “common” from a single number without asking which measure it is.

Worked numeric example (invented cohort, not an NBCE-published statistic): a college of 2,000 students is followed for one year. Twenty students develop new laboratory-confirmed giardiasis. Incidence = 20/2,000 per year = 1% per year (10 per 1,000 person-years). On June 1, six students still have symptoms or documented infection: point prevalence = 6/2,000 = 0.3%. If each illness lasts 3 weeks (≈0.058 years), expected prevalence ≈ 0.01 × 0.058 ≈ 0.06% if timing were uniform—the June 1 count being higher would make you ask whether an outbreak is still on, or whether duration is longer than 3 weeks.

Related measures:

Bias reminders that steal points: prevalence studies over-sample chronic survivors (length-biased sampling). A clinic chart review of “how many patients have tinea pedis today” is prevalence, not incidence. A summer camp that logs new diarrheal cases each day is incidence (and possibly an outbreak investigation).

Emerging diseases

An emerging infectious disease is new in a population or is rapidly increasing in incidence or geographic range. Re-emerging diseases return after control (measles in undervaccinated clusters; TB with HIV). Drivers you should be able to name:

Candida auris (2019 notifiable) is a model emerging fungus: skin colonization, environmental persistence, multidrug resistance, ICU outbreaks—control is contact precautions, cleaning with products that actually kill it (many quats fail; follow CDC/EPA lists), and reporting. SARS-CoV-2 is the model emerging virus of this decade: airborne/droplet debate settled toward inhalation, isolation of cases, vaccination, and genomic surveillance. Oropouche (bunyavirus, Culicoides midges and mosquitoes, Latin America and travel cases) is on the 2025–2026 notifiable list—an emerging arbovirus, not a helminth. Mpox showed that a zoonotic orthopoxvirus can sustain human-to-human spread in new networks. Do not treat “emerging” as a synonym for “viral”; fungi and parasites emerge too (drug-resistant malaria, Cryptosporidium in water parks, northern spread of tick and mosquito maps).

Prevention stack in a chiropractic clinic (applies the whole section): stay home when you have a febrile respiratory illness (isolation of the ill); hand hygiene between patients; low-level disinfection of tables (noncritical) with labeled contact time; sterile single-use needles if you use any critical item; know your state reportable list for measles, meningococcus, foodborne clusters, and (where listed) TB; refer immunocompromised patients with subacute headache or dry cough rather than assuming a mechanical lesion (crypto, PCP). Those behaviors are disease control. They are also how eukaryotic pathogens from the first two sections of this chapter fail to become your outbreak.