John Hay
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Human Anatomy & Physiology (Human Body Systems)
Unit 3: Unit 3: Transport & DefenseHBS 3.3Human Body Systems: virology & risk

Assessing pathogen risk

Estimate risk by combining how likely an infection is (likelihood) with how bad it would be (severity).

Builds on (2 levels back)inferred · high confidence
  • Routes of transmission: Likelihood of catching a pathogen depends on how it spreads (airborne, droplet, contact), so you must know transmission routes first.
  • Ordering levels from low to high: Risk uses ranked levels (low/medium/high), so you need to compare and order categories before combining them.

Prerequisites are inferred: pending teacher review.

Re-learn the skill with worked practice and clear examples.

Risk = likelihood x severity. Likelihood is how probable an infection is (driven by transmission), and severity is how harmful it is if it happens.

Step 1: Define both factors
Likelihood is how probable it is that a person gets infected, which depends on how the pathogen is transmitted. Severity is how serious the harm is if infection happens, from mild to life-threatening.
Step 2: Combine them
Risk is likelihood multiplied by severity. A high score on one factor alone is not enough; both have to be considered together.
Step 3: Watch the trap
A very deadly pathogen that almost never spreads can be lower risk than a moderate one that spreads easily, because risk multiplies the two factors.
Practice

Using Risk = likelihood x severity and the data shown, which pathogen has the highest risk score?

Reviewed
PathogenLikelihood (1-3)Severity (1-3)
P31
Q13
R33
S22
Four pathogens with likelihood and severity each rated 1 to 3. P is 3 and 1, Q is 1 and 3, R is 3 and 3, S is 2 and 2.
  1. A.Pathogen P
  2. B.Pathogen Q
  3. C.Pathogen R
  4. D.Pathogen S
Show the worked solution ▾

Answer: C. Pathogen R

  1. Step 1: Multiply each pair: P = 3 x 1 = 3. Q = 1 x 3 = 3. S = 2 x 2 = 4.
  2. Step 2: Multiply R: R = 3 x 3 = 9.
  3. Step 3: Compare: 9 is the largest score, so pathogen R has the highest risk.

Why it's right: Risk = likelihood x severity, so R scores 3 x 3 = 9, higher than P (3), Q (3), and S (4).

Why the others miss:
  • A: P scores 3 x 1 = 3 because its high likelihood is offset by low severity.
  • B: Q scores 1 x 3 = 3 because its high severity is offset by low likelihood.
  • D: S scores 2 x 2 = 4, which is less than R's 9.

Aligned to HBS 3.3: risk as likelihood x severity · reading level ~grade 9

Where you'd see this
  • A hospital ranks which incoming pathogens need isolation first by scoring likelihood times severity.
Careers that use this
Infection Preventionist: Scores pathogens by likelihood and severity to decide which need the strongest containment.
Video library
Watch: Assessing pathogen risk
How pandemics spread
TED-Ed · ~5 min
Guided notes

Fill these in as you work through the lesson.

Big idea: Risk is not just how deadly a pathogen is or just how easily it spreads, but the two combined: likelihood multiplied by severity.
Key terms: write the meaning
  • Likelihood (how probable):  
  • Severity (how serious the harm):  
  • Transmission (how it spreads):  
  • Risk (the two combined):  
The rule

Risk =   multiplied by  , so a pathogen that is both easy to catch and very harmful is the highest risk.

Check yourself
  1. Why is a deadly pathogen that almost never spreads not automatically the highest risk? 
  2. How does an airborne route change the likelihood compared with a pathogen that spreads only by direct blood contact? 
  3. If two pathogens have the same severity, what makes one higher risk than the other? 
Work one example

A pathogen spreads through the air (high likelihood) and usually causes severe illness (high severity). Use Risk = likelihood x severity to explain why it ranks as a high-risk pathogen.