Many people get ‘travel cover insurance’ free along with their credit card or bank account. However this may only give you very restricted ‘travel accident cover’, which may not, for example cover routine illness, or an accident that occurs whilst not actually travelling on public transport that was paid for using the card concerned.
You should read your card’s terms and conditions very carefully, and remember that, as in most things, ‘you get what you pay for’.
The benefit of having peace of mind whilst traveling in somewhere like Africa is well worth the cost of obtaining a proper comprehensive travel cover policy. Your tour operator may (and should) insist on being provided with evidence that you are fully covered by insurance.