For more than two decades the Zwicks have helped thousands of Latin American immigrants make the transition to life in the United States. What began as a humble shelter for refugees from war-torn Central America has evolved into a multifaceted operation that includes two health clinics, a labor hall and clothes- and food-distribution components. Casa Juan Diego, located on Lillian Street just south of Washington Avenue, houses as many as 150 immigrants a day, among them paraplegics, AIDS patients and others with serious health problems. Supported by donations, the center offers orientation and support to those who have recently arrived in the States. More than 100 day laborers assemble each morning at St. Joseph the Worker Labor Hall on Shepherd to seek jobs in construction, gardening, house-painting -- whatever comes around. The efficient operation provides contractors with much-needed labor while protecting the workers' right to a fair wage. Some 300 families receive rice, beans, corn, cabbage, tortillas and other staples at Casa's two food-distribution sites, while scores of uninsured patients get medical attention at the health clinics. "This is our life," says Mark Zwick of the couple's work.