TIJUANA, Mexico (AP) — Aid workers and humanitarian organizations expressed concerns Thursday about unsanitary conditions at the sports complex in Tijuana where more than 6,000 Central American migrants are packed into a space adequate for half that many people and where lice infestations and respiratory infections are rampant.

As a chill rain fell, the dust that coated everyone and everything in the open-air stadium turned to mud Thursday, making the already miserable conditions worse. On one side of the complex, a mud pit grew where people took outdoor showers next to a line of foul-smelling...