It can't come as a surprise that this region is an endemic malaria region. Lying by the lake it's superb breeding ground for mosquitoes and there's a good chance you'll catch malaria if you stay for more than a couple of months. I'm sure you know this already and have brought the right medication and profylaxis - if not, you can buy it in any duka all over Tanzania. Don't mean to break your spirits, but you will probably get malaria anyway. I did, and I was on Malarone, so use mosquito net and insect repellant - it's actually the best protection.
The mosquito carrying falciparum (brain malaria) is common in the area, so seek treatment the minute you start feeling any symptoms (fever, head ache, diarrhoea, vomiting, loss of appetite, aching joints etc.). Don't be too worried, though. If you do get malaria, Tanzania is probably the best place to go for treatment, as they see this every day and know exactly how to handle it. My best advice is to go straight to one of the private hospitals (DON'T go to Bugando Hospital, it'll take forever) and have a blood slide done. In a matter of minutes you will know if you have malaria or not, and get the proper treatment immediately. Don't expect to be hospitalized - that's only if you have a very serious case. If you discover it in time, you should be just fine.
The private hospital I used to go to, and which is one of the best in town (also has a European doctor) is Amaini Clinic which lies on the right hand side of the road leading down to U-turn. Otherwise, go to Aga Khan Hospital or Hindu Union Hospital.
At Amaini they will tend to give you Arinate, which is a very good treatment with very few side effects. It is what I always used when I had malaria. Otherwise, it is Fansidar, which is also very effecient but you can experience some heavy side effects. For younger children it is usually chloroquine. Remember to take a follow up blood slide when the treatment dose is finished. A blood slide typically costs 1000 sh in Mwanza.
There are a lot of street children in Mwanza, and they (or others) will rob you if you don't take a few precautions. First of all, never walk around with valuables in your hands. THey will hit you on the hand, grab whatever you have and run away. Secondly, and more important NEVER walk on the streets of Mwanza after dark. It sounds ridiculous, but even if you're walking only 200 metres from your hotel to a restaurant take a cab. As soon as it gets dark, the city changes and unless you're walking in a large group with big guys, you are sure to be robbed. The kids gang up on people and beat them or steal their things, and you can't expect help from others. You may think I'm exaggerating, but Mwanza is not a good town after dark, and when taxis are so cheap, it's just not worth risking getting mugged. Several of my friends have experienced getting robbed, so don't take the chance, but get friendly with the taxi drivers and take their phonenumbers so you can get a taxi any time. Around New Mwanza Hotel and down by the water are probably the worst places.
It's a problem all over Tanzania, but nevertheless it is important to be aware of. A lot of taxi drivers in Mwanza start drinking heavily after 7 pm, which results in them being extremely drunk at around 9. This doesn't stop them from driving, though and it can be difficult to find a sober cab driver. Still, it is probably more safe to have a moderately drunk taxi drive you back to your hotel than risk walking back. If you can't find someone sober, find the least drunken one, get in the back seat, use seat belt, and tell him that you know he is drunk and that you will be pissed at him if he doesn't drive carefully. Again, I can't recommend enough having 2 or 3 regular cab drivers whom you trust and can call up at any time. Also, if you're a girl and you're going with a new cab driver at night time, ask one of your friends to come with you or let the driver overhear that a friend is going to call you when you get to your place. When you get there, make the driver wait for you to get inside. Needless to say, arrange a price before you get in the cab, and avoid a nasty argument - especially if the driver is drunk it can get ugly.