I just wanted to chime in with some research I found on tracking undocumented (or unauthorized, or illegal - depends on the politics of the person using the term) immigrants. In some ways, this is a very tough question, as even the term "undocumented" itself suggests a difficulty in tracking a given population. According to a 2010 report from the Department of Homeland Security (http://www.dhs.gov/xlibrary/assets/statistics/publications/ois_ill_pe_2010.pdf), the "number of unauthorized immigrants living in the United States in January 2010 was 10.8 million—the same as in January 2009—but down from 11.8 million in January 2007. Between 2000 and 2010, the unauthorized population grew by 27 percent. Of all unauthorized immigrants living in the United States in 2010, 39 percent entered in 2000 or later, and 62 per-cent were from Mexico."
But this only gets at statistics of the undocumented (or as government sources tend to call them, "unauthorized" - good to keep in mind if you're doing future research on this topic) population as a whole. For some information on health issues, I found this organization, Migration Information Source (http://migrationinformation.org/about.cfm), which "provides fresh thought, authoritative data from numerous global organizations and governments, and global analysis of international migration and refugee trends. A unique online resource, the Source offers useful tools, vital data, and essential facts on the movement of people worldwide." The have a lot of data related to unauthorized immigration in the US and abroad, which might come in handy for future research. As a tip, when searching for documents on this site, remember to put commas between any terms that are of interest to you, rather than just a straight list (so, instead of what you might pop into Google, something like "unauthorized immigration US health" you'd need to structure it as "unauthorized immigration, US, health."
