The year is 1984. A small town in Oregon is about to experience something no American city had ever faced: a deliberate biological attack. You are an investigator. Your job is to uncover the truth.
Identify an outbreak using person, place, and time.
Interpret an epidemic curve and recognize a point-source pattern.
Calculate a simple incidence rate and attack rate.
Explain how interviews, lab evidence, and public health surveillance work together.
Describe why investigators should avoid jumping to conclusions before evidence is strong.
CONTENT NOTEThis activity discusses a real biological attack and illness outbreak. It does not include graphic content, but it does discuss intentional harm and criminal behavior.
IMPORTANT CONTEXTThis investigation focuses on crimes committed by specific leaders and participants. It does not ask you to judge every person associated with the broader religious community.
Read through each chapter of the story. Answer the investigation questions to earn XP and unlock the next chapter. Use the Field Manual, Epi Data, Notes, and Student Worksheet buttons to help you investigate.
Tip: download the worksheet before you begin if your teacher asks you to turn in your reasoning.
What was the strongest evidence that this was not a normal food poisoning outbreak?
What mistake could investigators have made if they jumped to conclusions too early?
How would this investigation be different today?
ANSWER REVIEW
📋 FIELD MANUAL — OUTBREAK INVESTIGATOR'S GUIDE
What Is an Outbreak Investigation?
An outbreak occurs when more cases of a disease appear than expected in a specific place and time. An outbreak investigation is the systematic process of finding out what caused those cases — who got sick, where, when, and why.
The 10 Steps of an Outbreak Investigation
1. Prepare for field work — Gather equipment, coordinate with agencies.
2. Establish the existence of an outbreak — Compare case numbers to baseline rates.
3. Verify the diagnosis — Confirm through lab testing that cases share the same illness.
4. Define and count cases — Create a case definition; find all cases.
5. Determine who is at risk — Describe the population by person, place, and time.
6. Develop hypotheses — What is the likely source? How was it spread?
7. Evaluate hypotheses — Use epidemiological studies to test your hypotheses.
8. Refine and re-evaluate — If hypothesis fails, go back to data.
9. Implement control measures — Remove the source, protect the community.
10. Communicate findings — Report to health authorities; share lessons learned.
What Is a Case Definition?
A case definition is a set of criteria used to decide if a person should be counted as a case in an outbreak investigation. It includes clinical features (symptoms), lab criteria, time, place, and population.
Example: "A case is any person who ate at a restaurant in The Dalles between September 9 and October 10, 1984, and developed diarrhea, nausea, or abdominal pain within 72 hours."
Types of Outbreak Investigations
Descriptive epidemiology: Who got sick? Where? When? (Person, Place, Time)
Odds Ratio (OR) — used in case-control studies: OR = (a × d) ÷ (b × c)
OR > 1 = exposure associated with increased odds of disease. For rare outcomes, OR ≈ RR.
Incubation Period
The time between exposure to a pathogen and the appearance of symptoms. Clues in the epi curve — the distance between the exposure date and the peak of illness — can suggest the likely incubation period and narrow the list of possible pathogens.
The Epidemiological Triangle
Three factors must be present simultaneously for disease to occur:
Agent — the pathogen or harmful factor
Host — the susceptible person
Environment — conditions that bring host and agent together
Bioterrorism manipulates the Environment corner — deliberately engineering the circumstances under which hosts encounter the agent.
Foodborne Illness Pathogens: Quick Reference
Each entry includes morphology, clinical presentation, and culture media used for laboratory identification.
Salmonella enterica (Typhimurium)
Type: Gram-negative bacterium
Morphology: Gram-negative rod (bacillus), 2–5 μm long, motile with peritrichous flagella. Non-spore-forming. Facultative anaerobe. Produces hydrogen sulfide (H₂S).
Morphology: Gram-negative rod (bacillus), 1–5 μm. Motile. Non-spore-forming. Facultative anaerobe. Sorbitol-negative (distinguishes O157:H7 from other E. coli).
Transmission: Undercooked beef, contaminated produce, unpasteurized juice
Symptoms: Bloody diarrhea, severe cramps, possible HUS (hemolytic uremic syndrome with kidney failure)
Incubation: 2–8 days
Culture Media:
Sorbitol-MacConkey Agar (SMAC): O157:H7 cannot ferment sorbitol — appears as colorless colonies (unlike other E. coli which ferment sorbitol and appear pink).
Morphology: Icosahedral capsid, ~27–40 nm diameter. No envelope. Extremely stable in the environment. As a virus, it cannot be grown on bacterial culture media and requires special approaches.
Transmission: Fecal-oral, contaminated surfaces and food, aerosolized vomitus
Symptoms: Vomiting (dominant), diarrhea, nausea — usually no fever
Incubation: 12–48 hours
Detection:
RT-PCR (Reverse Transcriptase PCR): Gold standard for detection from stool or environmental samples.
Enzyme Immunoassay (EIA): Rapid antigen test — less sensitive than PCR.
Note: Viruses cannot be cultured on standard bacterial media. Viral culture requires cell culture systems (living cells) or is replaced entirely by molecular testing.
Campylobacter jejuni
Type: Gram-negative bacterium
Morphology: Gram-negative curved or S-shaped rod ("spiral" or "comma-shaped"), 0.5–5 μm. Single polar flagellum giving rapid corkscrew motility. Microaerophilic (requires 5% O₂, 10% CO₂ — cannot grow in normal air or fully anaerobic conditions).
Transmission: Raw or undercooked poultry, unpasteurized milk, contaminated water
Symptoms: Diarrhea (often watery then bloody), cramping, fever
Incubation: 2–5 days
Culture Media:
Campylobacter Blood Agar (CAMPY-BAP) or Skirrow's Medium: Selective antibiotic-containing media incubated at 42°C (microaerophilic conditions, ~5% O₂). Growth in 24–48 hours.
Important: Will NOT grow on standard MacConkey or HE agar at room air — requires special atmosphere.
Transmission: Preformed enterotoxin in food left at room temperature; food handler contamination
Symptoms: Rapid-onset nausea, vomiting, cramps — usually no fever (toxin-mediated, not infection)
Incubation: Very short — 1–6 hours
Culture Media:
Mannitol Salt Agar (MSA): Selective (7.5% NaCl inhibits most bacteria) and differential (mannitol fermenters turn yellow). Staph aureus grows and ferments mannitol — yellow colonies on yellow background.
Blood Agar Plate (BAP): Golden colonies with beta-hemolysis (clear zone around colony due to complete red blood cell lysis).
Bacillus anthracis (anthrax)
Type: Gram-positive, spore-forming bacterium
Morphology: Gram-positive large rod (bacillus), 1–1.5 × 3–5 μm. Non-motile. Forms central endospores (highly resistant to heat, chemicals, and desiccation — can survive decades in soil). In culture, forms "medusa head" or "ground glass" colonies.
Symptoms: Depends on route — inhalation: flu-like then sudden hemorrhagic shock; GI: nausea, fever, bloody diarrhea, sepsis
Culture Media:
Blood Agar Plate (BAP): Large, gray-white, non-hemolytic colonies with irregular "comma" projections. Biosafety Level 3 (BSL-3) required.
Confirmation: PCR for protective antigen (PA) gene, direct fluorescent antibody (DFA) testing.
CDC Category: Category A (highest priority bioterrorism agent)
What Is Bioterrorism?
Bioterrorism is the intentional use of biological agents — bacteria, viruses, or toxins — against people, animals, or crops to cause harm for political, religious, or ideological purposes.
CDC Bioterrorism Agent Categories
Category A: Highest risk — easily spread, high mortality, major public impact. Examples: anthrax (B. anthracis), plague (Y. pestis), smallpox (variola).
Category B: Moderate risk — spread with some difficulty, lower mortality. Examples: Salmonella, brucellosis, ricin.
Category C: Emerging threats — could be engineered for mass spread. Examples: emerging viral diseases.
What Made the 1984 Attack Unusual?
Used a Category B agent (Salmonella) rather than a higher-risk agent
Involved a laboratory strain of Salmonella that was handled by perpetrators before the outbreak
Used food as the route of exposure
Relied on ordinary public settings rather than a military-style attack
Originally classified as an accidental food poisoning outbreak
Remained unattributed for over a year after the attack
Key Lesson for Public Health
The 1984 attack was not initially recognized as bioterrorism. This highlights the critical importance of syndromic surveillance — monitoring patterns of illness to detect unusual events — and maintaining a high index of suspicion when outbreak patterns don't fit a natural explanation.
Laboratory Procedures in Outbreak Investigation
Understanding how the lab identifies pathogens is essential for interpreting outbreak findings.
Gram Stain
The Gram stain is the first step in characterizing an unknown bacterium. It divides bacteria into two groups based on cell wall structure:
Gram-negative (−): Thin peptidoglycan layer with outer lipopolysaccharide (LPS) membrane — loses crystal violet, counterstained with safranin → appear pink/red. Examples: Salmonella, E. coli, Campylobacter (curved rods).
The Gram result, combined with cell shape (coccus=round, bacillus=rod, spirochete=spiral) and arrangement (clusters, chains, pairs), provides a rapid presumptive identification within minutes.
Note: Viruses are too small to be seen on Gram stain and are not detected by this method.
Culture & Sensitivity (C&S) for Enteric Pathogens
A stool culture grows bacteria from a fecal sample to identify the pathogen and test its antibiotic sensitivity.
Standard stool culture panels typically include:
MacConkey Agar: General Gram-negative selective medium. Lactose fermenters (E. coli) = pink; non-fermenters (Salmonella, Shigella) = colorless.
HE / XLD Agar: Selective for Salmonella and Shigella.
Campylobacter selective agar at 42°C in microaerophilic atmosphere (must be specifically requested).
Sorbitol-MacConkey (SMAC): For E. coli O157:H7.
Once a pathogen is isolated, an antibiogram (antibiotic sensitivity panel) is performed:
Antibiogram: A disk diffusion or broth microdilution test that measures bacterial growth in the presence of various antibiotics. Results reported as Sensitive (S), Intermediate (I), or Resistant (R) for each antibiotic. The antibiogram is what allowed investigators to confirm that the outbreak strain matched the Rajneeshpuram lab strain — both had identical resistance/sensitivity profiles.
Biochemical Panel
Once a pure bacterial culture is obtained, a biochemical panel (also called a biochemical identification battery) tests the organism's metabolic characteristics:
Can it ferment certain sugars? (glucose, lactose, sorbitol, dulcitol)
Does it produce hydrogen sulfide (H₂S)?
Does it produce indole? Urease? Oxidase?
Does it utilize citrate as a sole carbon source?
These results create a biochemical "fingerprint" that identifies the species. In the Rajneeshee case, the critical finding was that the strain did not ferment dulcitol — a characteristic found in only ~2% of Salmonella Typhimurium strains. This unusual biochemical profile helped link all 751 cases to a single source.
Modern automated systems (e.g., VITEK®, BD Phoenix™) can run dozens of biochemical tests simultaneously and identify organisms in 4–8 hours.
Molecular & DNA Testing
Molecular methods provide the highest specificity and are now standard in outbreak investigations:
PCR (Polymerase Chain Reaction): Amplifies specific DNA sequences to detect a pathogen's presence directly from a sample — no culture needed. Very rapid (2–4 hours). Used for Norovirus (RT-PCR), Salmonella, E. coli O157, and many others.
Pulsed-Field Gel Electrophoresis (PFGE): The traditional gold standard for molecular fingerprinting. Cuts bacterial DNA with restriction enzymes and separates fragments by size, creating a unique "bar code" pattern. Used to match the Rajneeshpuram strain to the restaurant cases in 1985.
Whole Genome Sequencing (WGS): Modern gold standard. Sequences the entire genome of the organism. Used in PulseNet today. Far more discriminating than PFGE — can distinguish strains that look identical by PFGE.
Plasmid Analysis: Plasmids are small, circular DNA molecules that exist separately from the main bacterial chromosome. They often carry antibiotic resistance or virulence genes. The identical plasmid profile across all 751 Rajneeshee cases was a critical forensic link. Bacteria from different sources rarely carry identical plasmids.
Plasmids
A plasmid is a small, circular, double-stranded DNA molecule that replicates independently of the bacterial chromosome. Key facts:
Can be transferred between bacteria (conjugation) — this is a major mechanism of antibiotic resistance spread.
Often carry genes for antibiotic resistance, toxin production, or metabolic functions.
The unique plasmid profile (size and number of plasmids) of a bacterial strain is like a fingerprint — identifying it as belonging to a specific population.
In the Rajneeshee case: the identical plasmid profile + the dulcitol non-fermentation trait + identical antibiogram = definitive match between the commune's stored bacteria and the restaurant outbreak strains.
Important Note on Viruses
Bacterial culture media (MacConkey, HE, Blood Agar, etc.) cannot support viral growth. Viruses are obligate intracellular parasites — they require living host cells to replicate. Options for viral detection include:
Molecular testing (PCR/RT-PCR): Most common in clinical labs today.
Cell culture: Inoculate living cell monolayers with specimen — observe for cytopathic effect (CPE). Rarely done for routine pathogens today.
Electron microscopy: Historical method; can visualize viral particles directly.
Antigen detection (EIA/RIA): Rapid tests using antibodies to detect viral proteins.
📝 INVESTIGATION NOTES
Write your observations, hypotheses, and findings here as you investigate. You can download your notes as a Markdown file when you're done.
📊 EPI DATA — THE DALLES OUTBREAK 1984
Case Summary
METRIC
VALUE
NOTES
Total cases
751
Salmonellosis confirmed
Hospitalizations
45
All survived
Deaths
0
No fatalities
Restaurants affected
10 of 36
In The Dalles, OR
Attack dates
Aug 29–Oct 10, 1984
Two main waves
Pathogen
S. Typhimurium
Gram-negative bacillus
Wave 1 cases (approx.)
~88
Sep 9–Sep 18, 1984
Wave 2 cases (approx.)
~586
Sep 19–Oct 10, 1984
Town population (1984)
~10,500–12,000
The Dalles, Oregon
Attack Rate Calculation Practice
Attack Rate Formula: AR = (Ill persons who ate food X ÷ Total persons who ate food X) × 100
Example from this outbreak:
Suppose 400 people ate at Shakey's Pizza. Of those, 320 developed salmonellosis.
AR = (320 ÷ 400) × 100 = 80%
A high attack rate in exposed individuals compared to unexposed individuals (those who did NOT eat at that restaurant) points to the food source.
Incidence Rate in The Dalles
Incidence Rate = (751 cases ÷ 10,500 population) × 1,000 = 71.5 cases per 1,000 people
This is extremely high for a single outbreak in a short period. A normal annual incidence of salmonellosis in the US is roughly 15–20 per 100,000 — this outbreak was about 500 times higher.
Epi Curve: Outbreak Timeline
FIGURE 1 — APPROXIMATE CASE DISTRIBUTION, THE DALLES OUTBREAK 1984
Wave 1 (~88 cases)Wave 2 (~586 cases)
Note: This is an approximate representation based on published data. Both waves show a point-source pattern — consistent with deliberate, repeated contamination events rather than person-to-person spread.
Key Lab Finding
All case isolates were Salmonella enterica Typhimurium. Importantly:
The strain did NOT ferment dulcitol — a characteristic found in only 2% of nontyphoidal Salmonella
All restaurant isolates had an identical plasmid profile — suggesting a single source
The strain matched perfectly with the bacteria stored in the Rajneeshpuram lab
📚 REFERENCES
All sources used in this educational exercise are from academic, governmental, or established news organizations.
Primary Sources (Provided)
Turner MD, Marinconz K, Shimp G. (July 22, 2025). The 1984 Rajneeshee Bioterrorism Attack: An Example of Biological Warfare by Violent Non-state Actors. Cureus, 17(7): e88514. DOI: 10.7759/cureus.88514
Najera RF. (December 14, 2024). The 1984 Rajneeshee Bioterror Attack Was Too Easy to Carry Out. Could It Happen Again? Microbial Instincts / Medium.https://medium.com/microbial-instincts/
WBUR / Associated Press. (2002). Bioterrorism in History: 1984 Rajneesh Cult Attacks Local Salad Bar. Archived via Wayback Machine. Archive link
U.S. Naval Postgraduate School / Defense Technical Information Center (ADA462699). The Rajneeshee Cult — Capabilities-Based Assessment of Bioterrorism.
Supporting Academic Sources
Torok TJ, et al. (1997). A large community outbreak of salmonellosis caused by intentional contamination of restaurant salad bars. JAMA, 278(5), 389–395. JAMA link