Make sure you are insured for health, accident, travel and third part liability.
Volunteering is NOT necessarily covered by your host's insurance and is undertaken at YOUR OWN RISK. Be sensible and responsible and do not undertake activities that you consider to be dangerous or outside your competence to perform safely.
Concerning health insurance, If you live in Europe, make sure you obtain an European Health Insurance Card (ex.: E 111) before you leave home. You will get it for free from your health insurance office. An EHIC gives you a degree of medical coverage in an emergency. In Greece, the EHIC gives you access to IKA state insurance service.
WWOOFers are always strongly advised that they must have insurance when they go WWOOFing.
Among others, O V Europa ( www.oveuropa.com ) is an insurance company that has been providing insurance for WWOOFers.
Think you don't need it? Think again!
Most WWOOFers have travel insurance and think they're covered. However, most of them don't check the fine print. Read carefully and you'll often find that any sort of work, including voluntary work, is excluded from the policy. This WWOOFer insurance doesn't replace your travel insurance but at only €25 a year it provides an essential complement for anyone who's WWOOFing on their travels.