Stay safe / healthy
Chad is consistently engulfed in political turmoil and attacks from rebels will probably not happen, but are certainly possible. The situation has stagnated, but it remains a threat. Violence from the Darfur conflict overspills into Eastern Chad from Sudan, a country which shares hostilities with Chad. Any activity outside of N'Djamena is done with difficulty at best. Northern Chad is barren, scorching desert and guides (good luck) and meticulous planning are required. In 2013, Boko Haram jihadists were spotted in Chad.
N'Djamena is RELATIVELY safe, although one should be wary of petty street crime and corrupt police/officials. Most border crossings are extremely difficult (Sudan and Libya not being a viable option) although the border crossings with Niger and Cameroon are relatively painless.
Don't accept water from any stores unless you know the brand. Eat only your own food that you buy in grocery stores. Avoid restaurants whenever possible. Stay away from people that look sick; there are many diseases in Chad to be aware of. If you are in Chad for a while, go to a doctor once a month if you can afford it.