Niger is one of the safest countries in West Africa.
In Niamey you should feel safe, even after dark, no matter what US Department of State says. In markets there is risk of pickpockets.
Carrying a backpack and camera, looking like a tourist, and especially being a white, will definitely draw some unwanted attention. Most of the attention is from people who try to get your money legally, either by selling you a toothbrush or by begging, but there are always few less honest people.
In the region north of Agadez, there have been many car-jackings, kidnappings and robberies in the past sixteen or so years. The problem continues to this day (most recently, 2 Italians were kidnapped by Tuareg rebels in September 2006), and tourists should consider the area essentially lawless.
Avoid driving late at night in a private vehicle. Ocassionally armed robbers will operate near the town of Galmi (central Niger) and around Dosso-Doutchi (in western Niger), as well as on the road to Gao, Mali in the Tillabery region. Normally, there are police check-points on the main highways which limit criminal activities during the day.
The Centers for Disease Control is an excellent resource for authoritative advice on health issues for travelers to Niger.
Drink lots and lots of water while in Niger because the dry heat will dehydrate you and you won’t realize it. It is the best preventative step you can take. Bottled water is available in most of the cities but in a pinch, city tap water is well-chlorinated (this is according to one traveler; another American who lived in Niger for two years says never drink unfiltered water anywhere! – that includes ice!). Be particularly wary of well water as it can be dirty.
Wear loose clothes, big hats, and lots of sunscreen. If in doubt, wear what the locals wear.
Malaria, including encephaletic malaria, is a problem, and is chloroquine resistant in Niger.Take your prophylaxes, use heavy-duty insect repellent (DEET is best, though nasty), and consider carrying a mosquito net (although they are horribly hot). If you are in a well-screened room, simply spray the room with readily-available insecticide 15 minutes before entering (like when you leave for a meal) and all your existing mosquitos and other critters should be dead.
Giardia, and much worse — dysentery — is also a problem. Be wary of any roadside food, unless you buy it hot off the grill. Even items fried in oil could make you sick if the oil has been heavily used and is old. If ordering a salad, asked if it was washed in eau de javel (household bleach) as that will kill anything nasty. Also, never drink unfiltered water (and no ice!).
Schistosomiasis is present in most water bodies in Niger, so travelers should avoid going in the water everywhere — except chlorinated swimming pools.
In case you were unable to stay healthy, the Clinique Pasteur (situated in front of the Lycée Fontaine) has clean facilities, sterile needles, and competent, sympathetic doctors. The Clinique Gamkalley and many other clinics are around, however, you may need to watch out for dirty needles, over-prescription and aggresive staff.
Visitors are treated as gods in Niger (actually the Hausas have a proverb to that effect), so be careful not to abuse the hospitality you will be shown. For the most part, try to accept all the small tokens and gestures (cokes, tea, small gifts, etc) that are offered to you during your time in Niger. It really isn’t good to refuse too much and don’t think these people are too poor to give me these things. That is offensive as taking good care of guests is a point of honor and gives people great pleasure. Don’t comment out loud when you see poverty or things in disrepair and please don’t remind Nigeriens about how poor their country is.
Also, remember Niger is a predominantly Muslim country. Covering up knees (especially women, but men too) is typically very appropriate and appreciated anywhere you go in Niger.