Sierra Leone requires a yellow fever vaccination certificate for entry — you'll be asked to show it at the border. Make sure you have it. Beyond that, the standard West Africa vaccine recommendations apply: hepatitis A, typhoid, polio, and rabies (if you plan to spend time in rural areas or with animals). Consult a travel health clinic at least four to six weeks before departure.
Malaria is present throughout the country year-round and is the most serious health risk. Take antimalarial prophylaxis — atovaquone/proguanil (Malarone), doxycycline, or mefloquine are the options; discuss with your doctor which suits you. Use DEET-based repellent, sleep under a treated mosquito net, and cover up after dark. If you develop a fever after returning home, tell your doctor you were in Sierra Leone.
Water: Don't drink tap water. Bottled water is widely available in Freetown and the main towns; up-country you may need to purify your own. Avoid ice in drinks unless you're confident of the source.
Medical facilities: Healthcare in Sierra Leone is severely limited. Freetown has a handful of clinics that expats and NGO workers use; quality is highly variable. Serious illness or injury may require medical evacuation. Get comprehensive travel insurance that includes emergency evacuation coverage before you go.
Ebola: The 2014-2016 outbreak was devastating for Sierra Leone. The country has been declared Ebola-free since then and has improved epidemic response systems, but it's worth knowing the history.