Rural workers (including farmers) and workers in the informal sector account for more than 90% of the workforce in Africa, and most have poor access to health care. In most African countries, health insurance is reserved for employees of the public and private sectors. There is no health insurance for the rest of the population.
Many different arrangements have been designed to address this situation.
Here’s a short piece about mutual health societies: http://www.ilo.org/public/english/region/afpro/abidjan/publ/ilo8/social7.pdf 
This website describes a Nigerian program which provides healthcare to low-income communities in two southern provinces for an annual fee: http://hygeiagroup.com/hchp/ 
Here are two FRW stories about insurance for farmers:
Uganda: Microinsurance schemes to help farmers deal with unforeseen circumstances (FRW #56, February 2009.) http://weekly.farmradio.org/2009/02/23/1-uganda-microinsurance-schemes-to-help-farmers-deal-with-unforeseen-circumstances-written-by-joshua-kyalimpa-for-farm-radio-weekly-in-kampala-uganda/ 
Kenya: Livestock insurance will protect livelihoods from drought and floods  (FRW #89, November 2009)
These Notes to broadcasters on microinsurance from FRW #56 may also be helpful:
Talk to farmers, extension workers, government workers, farmers groups, health NGOs and private or public insurance companies. Find out what kinds of health or other insurance are available for farmers and other rural people. What do people do in the absence of health insurance? Do they have health insurance through farmers’ groups? Or perhaps they have home-grown solutions such as rotating savings and loans clubs, loans from relatives, etc.?