While it's true that observational studies can't prove causation on their own, the overwhelming body of evidence—包括 decades of clinical trials, treatment response, and the consistent biological mechanism—supports the causal link between HIV and AIDS. When people stop effective antiretroviral therapy, their viral loads rebound, and AIDS develops in the absence of treatment, which is a strong indicator of causation. This isn't just correlation; it's a direct biological pathway that's been validated across multiple studies and real-world outcomes.

The San Francisco study you mention is part of a broader body of research that includes randomized controlled trials, which have consistently shown that HIV leads to AIDS if left untreated. The analogy to a car engine isn't perfect, but it's a useful simplification to illustrate how a virus can systematically damage the immune system over time. The fact that some people don't develop AIDS despite HIV infection is well understood—it's due to genetic factors, immune responses, and other variables, not the absence of a causal relationship. These exceptions don't negate the overall causation; they just highlight the complexity of individual variation.