Conservation work is not usually a particularly hazardous activity. However it does increase the risk of exposure to a few nasty illnesses, in particular Tetanus, Weil's Disease and Lyme Disease. Some of the plants we work with also present unusual hazards.
We must admit that the occasional volunteer has got sunburnt on a task with us, but as far as we know, nobody out of the hundreds of volunteers who have come out with LCV in the last 40 years has ever caught any of the more serious diseases.
With luck your client should point out the hazardous species / substances on the site but there are some generally sensible precautions which can be taken to minimise the small chances of suffering from any problems as follows:
It is also advisable that you can recognise and are familiar with some of the more common health hazards of conservation work. More detailed advice about these can be found by following these links.