Sentence improvement — Choose the most accurate verb for a gas causing illness: improve the report ‘‘a mysterious nerve gas affected a large number of people’’ to a more precise verb that aligns with health-related usage.

Given data
Original: ‘‘Not long back, in Japan, a mysterious nerve gas affected a large number of people.’’

Concept/Approach
In health contexts, when an agent causes disease/illness in people, the verb ‘‘infect’’ is commonly chosen over the general ‘‘affect’’ or the incorrect ‘‘effect’’ (which means ‘‘bring about’’). ‘‘Infest’’ applies to parasites occupying a place, not to people being made ill.

Option analysis
effected — Means ‘‘brought about’’; wrong object and meaning.infected — Best medical/health verb for causing illness among people.infested — Used for parasites/places, not for people suffering illness.No improvement — Keeps the vague ‘‘affected’’.impacted — Colloquial and non-specific.

Verification/Alternative
Depending on style, ‘‘poisoned’’ could also be used for toxic gas exposure; in the provided options, ‘‘infected’’ is the closest precise alternative.

Common pitfalls
Confusing ‘‘affect’’ vs ‘‘effect’’; misusing ‘‘infest’’ for human subjects.

Final Answer
infected