Most expatriates moving to Sierra Leone will have their companies provide international medical insurance. The usual deductible or co-payment (depending on your insurance type) applies. Hospitals generally have reputations for being overwhelmed and understaffed, but there are some doctors who are fine to see for a general diagnosis.
Any condition that is serious or requires ongoing treatment is typically treated in the expat's home country. If an emergency arises that calls for immediate care, medical evacuation is generally used.