I would second Mary Maniery's list of archaeological health hazards and add
West Nile Virus, which will be in California by next summer (if it isn't
already). It is also good to have basic immunizations (tetanus and specific
medications for your particular region). For work in a cholera area in Brazil, I
found nothing except being careful what went into my mouth and washing hands in
PURE water constantly could help. It is much more difficult to maintain a safe
environment in the field than in an international hotel.
Bob Hoover