Which influenza strains affect humans?

• A. Type A and B
• B. Type C only
• C. Type D
• D. All strains equally

Answer: (A)

Influenza in humans is mainly caused by two types that circulate each season: Type A and Type B. Type A can infect many species and undergoes changes that drive both common seasonal outbreaks and occasional pandemics, which is why it’s a major human pathogen. Type B also causes seasonal flu in humans, but it doesn’t have the same broad animal reservoir or the same level of genetic diversity as Type A, so it stays within humans and tends to be less variable but still clinically significant.

Type C can infect humans but typically leads to mild illness and does not drive seasonal epidemics. Type D primarily affects cattle and isn’t known to regularly infect humans, so it isn’t considered a human influenza strain.

So, the strains that effectively cause flu illness in humans on a seasonal basis are Type A and Type B. The others either cause only mild disease in humans or rarely, if ever, infect humans, which is why they aren’t described as the human influenza strains.