All hosts should have insurance. Most take out public liability and employers liability, but WWOOF cannot explicitly advise you what you need.
WWOOFers - you are responsible for your own safety. Check you have insurance that covers medical care including transportation home, accidents, and loss of property. You should also be covered against you injuring other people or losing or breaking other peoples' property. Make sure your insurer knows that you will be a volunteer on a farm.
In the UK we have something called the European Health Insurance Card (EHIC) that you can apply for from main Post Offices and it covers you needing state medical care in any country in the EU. There may be something similar available in other countries.
If you live in Europe, or anywhere else, you can also apply to OvEuropa (www.oveuropa.com) for insurance cover. This registered non-profit association provides its members with a third party liability and personal accident, injury and illness insurance suited to volunteer work. The association was formed to respond to the need for a suitable insurance at a low cost for WWOOFers staying with WWOOF hosts. The association can also provide information about visas, permits, health and medical care requirements within Europe.
The cost of joining OvEuropa is 35.00 Euros for one year for their basic plan, including the insurance premium.