Zoonoses and evaluating COVID-19 dataSpec C3.2.15, C3.2.18
In short
Zoonoses are infectious diseases that can transfer from other species to humans. They are common and spread in varied ways: tuberculosis from cattle, rabies through bites from infected mammals, Japanese encephalitis through mosquitoes, and COVID-19, which recently transferred from another species. Evaluating COVID-19 data involves calculating percentage change and percentage difference.
Zoonoses
Zoonoses are infectious diseases that can transfer from other species to humans. Many human infectious diseases are zoonoses, and their modes of infection are varied.
| Disease | Pathogen | Mode of infection |
|---|---|---|
| Tuberculosis (bovine) | Bacterium (Mycobacterium bovis) | From cattle, through drinking unpasteurized milk or breathing in droplets from infected animals. Most human tuberculosis is now spread from person to person by a related species, Mycobacterium tuberculosis. |
| Rabies | Virus | Through a bite from an infected mammal, such as a dog or bat, as the virus is in the saliva |
| Japanese encephalitis | Virus | Through the bite of a mosquito that has fed on infected pigs or wading birds |
| COVID-19 | Virus (SARS-CoV-2, a coronavirus) | Its closest known relatives are coronaviruses found in horseshoe bats. Most evidence points to spillover from animals, possibly through an intermediate host, but the exact route into humans has not been confirmed. It now spreads between humans in respiratory droplets and aerosols. |
COVID-19 is an infectious disease that has recently transferred from another species, with profound consequences for humans: millions of deaths worldwide, overwhelmed health services, and major social and economic disruption. The IB guide treats it as a zoonosis; how the virus first reached people is still being investigated, so avoid stating a single proven source in answers.
Evaluating COVID-19 data
Pandemic data (cases, hospital admissions, deaths, vaccination rates) are often compared over time or between groups. Two calculations are used:
Percentage change compares one quantity at two times, so it has a starting value to divide by and a sign (+ for an increase, − for a decrease). Percentage difference compares two values where neither is the starting point, for example two groups or two countries at the same time, so you divide by the mean of the two and the answer is always positive. If a question says 'relative to' one value, divide by that value instead and say so.
Percentage change and percentage difference
Practice data: in one city, weekly COVID-19 hospital admissions fell from 250 to 90 over two months. In the final week, the admission rate was 12 per 100 000 among unvaccinated adults and 3 per 100 000 among vaccinated adults. Calculate the percentage change in admissions, and the percentage difference between the admission rates of the two groups.
- Percentage change = (90 − 250) ÷ 250 × 100
- = −160 ÷ 250 × 100 = −64%
- Mean of the two rates = (12 + 3) ÷ 2 = 7.5 per 100 000
- Percentage difference = |12 − 3| ÷ 7.5 × 100 = 9 ÷ 7.5 × 100 = 120%
- Relative to unvaccinated adults only: (3 − 12) ÷ 12 × 100 = −75%
Answer: Admissions fell by 64%. The percentage difference between the two admission rates is 120%; put another way, the rate in vaccinated adults was 75% lower than in unvaccinated adults.
When evaluating pandemic data, consider sample size, how cases were tested and recorded, whether the groups differ in age or health, and that correlation does not prove causation. Linking question: false-positive and false-negative test results can distort case numbers.
Quick check
Which four groups of organism is the term pathogen usually reserved for?
Show answer
Viruses, bacteria, fungi and protists.
Which enzyme converts fibrinogen into fibrin?
Show answer
Thrombin.
What two things are needed to activate a B-lymphocyte?
Show answer
Direct interaction with its specific antigen, and contact with a helper T-cell activated by the same antigen.
Why do antibiotics not work against viruses?
Show answer
Viruses have no metabolism of their own, so there are no bacterial processes to block; they use the host cell's processes.
Name three zoonoses given in the IB guide besides COVID-19.
Show answer
Tuberculosis, rabies and Japanese encephalitis.
Written and checked against the IB Biology SL specification · Updated October 2026