France is a healthy place, so your main risks are likely to be sunburn, foot blisters, insect bites and mild stomach problems from eating and drinking with too much gusto.
Citizens of the EU, Switzerland, Iceland, Norway or Liechtenstein receive free or reduced-cost state-provided health-care cover with the European Health Insurance Card (EHIC) for medical treatment that becomes necessary while in France. Each family member will need a separate card. UK residents can get application forms from post offices, or download them from the Department of Health website (www.dh.gov.uk), which has comprehensive information about the card's coverage.
The EHIC does not cover private health care, so make sure that you are treated by a state health-care provider (conventionné). You will need to pay directly and fill in a treatment form (feuille de soins); keep the form to claim any refunds. In general, you can claim back around 70% of the standard treatment cost.
Citizens of other countries need to check if there is a reciprocal arrangement for free medical care between their country and France.