The virus, spread by mosquito bites, will prevent Alfonzo from flying, according to the club. He's expected to return from Venezuela within a week, but when he does, he will report to Minor League camp and not Major League camp.
Symptoms include a high fever and skin rash and usually last from one to two weeks. The illness is not considered life threatening. Alfonzo will be replaced in Major League camp by Minor League catcher Matt Wallach, son of third-base coach Tim Wallach.
Alfonzo, 33, was suspended twice for testing positive for performance-enhancing drugs, but the second suspension, for 100 games, was overturned for procedural reasons.
Alfonzo hit 12 homers for the Giants as a rookie in 2006, and made San Francisco's Opening Day roster in 2007, but has since been a member of the Padres, Mariners and Rockies organizations before playing in Mexico last year.