Alabama farmer Mac Barlow spent decades using a weed killer to clear his fields. Now diagnosed with Parkinson’s, he is among thousands who say this pesticide may be to blame for their health problems. His story highlights an increasingly urgent debate on chemical use and public health.

Michael J. Fox, best known for his role in “Family Ties,” has spoken about his journey with Parkinson’s disease, diagnosed at age 29. Now, after 35 years battling the condition, he shares his wish for a serene end, rather than a dramatic one.