Passage Summary: Some people think a certain stomach bacterium is 'good' because it only makes a few people sick and might help the immune system. The author disagrees, pointing out that we don't call the tuberculosis germ 'good' just because most people who have it don't get sick.

Conclusion: It is incorrect to classify the bacterium H. pylori as a beneficial commensal.

Reasoning: Although it only causes ulcers in 10 percent of cases, we do not call tuberculosis a commensal even though it also only makes 10 percent of its hosts sick.

Analysis: The author relies on a comparison between two different bacteria to make their point. To weaken this, we need to find a way to show that H. pylori and tuberculosis aren't actually comparable in this context. If H. pylori provides a massive benefit to the 90% of people who don't get sick—a benefit that tuberculosis definitely doesn't provide—then the author's analogy falls apart. Look for an answer that highlights a positive trait of H. pylori that the author has conveniently ignored.