This was originally posted as a comment on Scott Alexander’s blog. Scott recommended the comment in Open Thread 420. I reproduce it here with edits.

Scott Alexander: Do some cancers prevent Alzheimers? There’s some evidence that people with cancer are less likely to develop Alzheimers (even adjusting for age/mortality/etc). Why? Some cancers produce large amounts of weird chemicals. One of those chemicals, cystatin c, appears to reverse Alzheimers in mouse models, maybe by dissolving amyloid plaques. And here’s me asking Claude some of the obvious followup questions.

Trade-offs between cancer and degenerative disease crop up fairly often. Both sets of conditions are thought to be consequences of an increased cheater-cell load in old age. Cancers,