Biotechnology for Health (Biomedical Innovations)

Unit 5: Problem 5: Combating a Public Health IssueBI 5.1Biomedical Innovation: epidemiology & public health

Compute incidence and prevalence

Tell apart two key public-health numbers: how many NEW cases appear in a period (incidence) and how many people have the illness right now (prevalence).

Builds on (2 levels back)inferred · high confidence

Writing a part over a whole as a fraction or percent: Both incidence and prevalence are a count of people divided by a population; you must be able to turn 'X out of Y' into a fraction and a percent.
Separating new items from items already there: Incidence counts only new cases while prevalence counts all existing cases, so you first have to split 'just appeared' from 'already present'.

Prerequisites are inferred: pending teacher review.

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

Incidence = new cases in a period ÷ population at risk. Prevalence = all existing cases at one time ÷ total population. Morbidity means being ill; mortality means death.

Step 1: Compute incidence

Count only the cases that newly appear during the period, then divide by the population at risk. Example: in a town of 1,000, 10 people newly get an illness this month, so incidence is 10 / 1,000 for the month.

Step 2: Compute prevalence

Count every case that exists at that moment: both old and new: then divide by the total population. Example: same town of 1,000, where 50 people already had it plus the 10 new ones, so 60 people have it now and prevalence is 60 / 1,000.

Step 3: Keep morbidity and mortality straight

Morbidity is about illness (how many are sick); mortality is about death (how many died). Incidence and prevalence are morbidity measures unless you are specifically counting deaths.

Practice

A town has 1,000 people. At the start of the month 50 people already have an illness, and during the month 10 more people newly catch it. What is the incidence for the month?

Reviewed

A.10 / 1,000 = 1%
B.50 / 1,000 = 5%
C.60 / 1,000 = 6%
D.10 / 60 ≈ 17%

Show the worked solution ▾

Answer: A. 10 / 1,000 = 1%

Step 1: Pick only the new cases: Incidence counts new cases. The 50 were already sick; only 10 newly caught it this month.
Step 2: Divide by the population at risk: 10 new cases out of 1,000 people: 10 / 1,000 = 0.01 = 1% for the month.

Why it's right: Incidence uses only the 10 new cases over the 1,000 population: 10 / 1,000 = 1% for the month.

Why the others miss:

B: 50 / 1,000 = 5% counts the people who were already sick, which belongs to prevalence, not incidence.
C: 60 / 1,000 = 6% is the prevalence (all 60 existing cases), not the incidence.
D: 10 / 60 wrongly divides new cases by the number of sick people instead of by the whole population.

Aligned to BI 5.1: incidence vs. prevalence · reading level ~grade 9

Where you'd see this

A health department tracks monthly incidence to see if new infections are speeding up, and prevalence to plan how many beds the current sick population needs.

Careers that use this

Epidemiologist: Reports incidence to judge how fast an outbreak is growing and prevalence to size the current burden on the health system.

Video library

Watch: Compute incidence and prevalence

Intro to Epidemiology: Crash Course Public Health #6

CrashCourse · ~13 min

Guided notes

Fill these in as you work through the lesson.

Big idea: Incidence counts only the NEW cases that appear during a period, while prevalence counts ALL the cases that exist at one moment: both written as a count divided by the population.

Key terms: write the meaning

Incidence (the brand-new arrivals in a time window):
Prevalence (everyone who has it right now, new or old):
Morbidity (the illness side: being sick):
Mortality (the death side: counting deaths):

The rule

Incidence = cases in a period divided by the population at risk. Prevalence = existing cases at one time divided by the total population.

Check yourself

In your own words, what is the one word that separates an incidence count from a prevalence count?
A report says 'morbidity is rising but mortality is steady.' What is happening to illness, and what is happening to deaths?
If 8 new people get an illness this month in a town of 800, set up the incidence fraction (do not simplify yet).

Work one example

A neighborhood has 500 people. At the start of June, 20 people already have the flu. During June, 5 more people catch it. Set up and compute the prevalence at the start of June and the incidence during June, showing each fraction and percent.