Not always. For instance, this study found the worm problem worse on the U.S. side of the border: "When comparing infection prevalence on either side of the border, our study findings suggested that he prevalence of Taenia spp. was significantly higher on the U.S. side of this study population, probably due to the use of mass deworming campaigns among schoolchildren in Mexico that have proved successful."The corruption of Mexican sewers make them more germ-ee, I think.
http://www.publichealthreports.org/issueopen.cfm?articleID=2408
When was the last time you had a mass deworming campaign in your neighborhood?