You will need to take out travel insurance appropriate to the countries you are visiting and what you will be doing when you get there.
Policies are generally either for an individual trip or on an annual basis. They may also be lumped together with other insurance products such as house or life insurance, or with bank accounts. The level of insurance cover, and therefore cost of the premium, will be affected by the location you are travelling to, the perceived risk to yourself or your belongings, the value of your belongings and the potential cost of any rescue or repatriation.
An important aspect of travel insurance is cancellation cover. Sometimes this is an optional extra on the basic policy and it can be tempting to save money by omitting it from the policy. We would always advise that you do take out the cancellation option so that if, for whatever reason, you are unable to travel you are able to claim back the costs of your bookings and flights. Your insurance policy should be taken out immediately after placing bookings and buying flights. In this way the money that you have just spent is immediately covered. If you were to suffer any issues (that meant you were unable to travel) between buying flights and buying insurance you would be unable to claim back the air fare.
When looking for insurance the policy make sure it covers all the regions you will be travelling in and includes all the activities that you will be involved in. Note that some of the more basic policies may exclude 'volunteering work', but you need to explain to the company what you are actually doing. Make sure they don't think you will be working on big construction sites etc. It's also important to ensure that the policy covers rescue and repatriation from the specific area and locations. These may be complicated by terrain or altitude so you need to be specific about where you will be and what you will be doing. Most companies will show lists of activities that fall into different categories of cover. You will need to read these carefully and preferably contact them to explain the exact nature of your trip and any expensive equipment you have.
If you can get them to email you back with words to the effect of "We can confirm that these are the details of a policy that will cover you for your trek to X and climb of X to a maximum altitude of X including helicopter search and rescue, also including X activities and covering your shiny new X to the value of X", where relevant to your trip.
You need to make sure that you fill out the medical declaration accurately and disclose any conditions or history that may be relevant, however small. In the unlikely situation where you need to make a claim, any undisclosed issues that may have affected your risk may cause your policy to be invalid and you can quickly rack up huge medical, rescue and repatriation costs.
If you have any concerns or queries regarding your insurance cover you should raise them specifically, in writing, with the insurance provider.
A few companies that might be worth contacting include:
There is also other advice available online, such as the Travel Insurance Guide.
There are providers who specialise in cover for people with existing medical conditions. Some general providers may either not provide cover or offer much higher premiums. If you do have specific requirements or conditions it is worth contacting companies such as All Clear Travel.