Yellow fever vaccinations are required if you are travelling from endemic areas/ countries. Precautions against malaria, bilharzias and cholera should be taken and medical advise taken regarding vaccinations. Malaria, especially of the falciparium variety is a common hazard throughout the year, so take adequate precautions in the form of anti-malaria medicines, mosquito repellent creams and mosquito nets, if possible. Water borne diseases like gastroenteritis, cholera and tummy upsets are common. Treat all water as potentially risky, unless it has been boiled, treated with water purifying tablets or comes from a sealed bottle. While medical services in the capital are adequate, facilities in the interior and upcountry leave a lot to be desired. Health insurance is essential.
Senegal is far safer than a lot of the volatile African nations but certain areas like the Basse Casamance and the southern regions need to be avoided when rebel activity hots up. Then being caught in the middle of a skirmish, abducted or taken hostage are very real dangers. Check with your embassy before travelling to these areas. Avoid going out alone late at night in the cities and towns, as street crimes and mugging are frequent occurrences. Tourists in Dakar, particularly, are at risk.