Plastic bags get caught up in the Acacia tree thorns, making it very difficult to get out of the trees. This tree was a landmark in our neighborhood. It was used to give directions, " When you come to the tree withall the plastic bags in it, turn left."
With no real garbage disposal system to speak of, Niamey has a serious problem with garbage, specifically plastic bags. It is a serious problem; on the outskirts of town you will run across huge areas where the trash from the city has been dumped. The wind whips the bags into the air and carries them into farmers fields.
Although Niger still has a very low reported incidence rate of AIDS, many are suspected of dying of the disease without ever seeking medical help. The government has started AIDS awareness campaigns. Hopefully it will help.
As I've said in other places, Niger is hot. On top of this, it is dry, so you don't notice your water loss through sweat like in moister climates. By the time you feel thirsty in this climate, it is already too late, you have already become dehydrated. Always carry a water bottle and drink even when you are not thirsty. This will save you from heat exhaustion, not a fun experience!
In general, we felt very safe in Niamey the three years we were there. We had no incidences of any kind with theft, etc. But things do happen. When rich tourists (and all tourists are rich) come to desperately poor countries like Niger, and then flaunt money, camera equipment, or cars, there are bound to be problems.
For this reason, it is not recommended to walk on the Kennedy Bridge or the area around the Hotel Gaweye.
Take the normal precautions with your valuables you would anywhere.
Car jackings are probably the biggest problem. These don't usually take place in Niamey, but in the desert, notably Agadez. Several people we knew had this happen there. How to prevent it? Well, you can't, but driving an older beat up looking car is less likely to be tempting than a brand new Land Rover.