ERIN, Wis. — Jimmy Walker’s wife cringed when, on her social media feed, she saw photographs of players and caddies wading into Erin Hills’s thick fescue grass during practice rounds for the United States Open . She remembered thinking, “Somebody’s going to get sick this week.” Her husband returned to competitive golf on Thursday at Erin Hills after a month off to recover from Lyme disease, a tick-borne bacterial infection that sapped his energy and sank his buoyant personality. Walker, who won the P.G.A. Champion ship last July, endured several months of brain fog, a shortened fuse and extreme fatigue before receiving a diagnosis in April. “I was pretty sick there for a while,” Walker, 38, said. “I did not feel worth a flip.” Erin Hills’s fescue, knee high in some places, is part of the challenge here for the players, but it is also a hospitable spot for bugs and a potential health hazard for everybody who treads on it, including caddies, volunteer marshals, securi...