Malawi is a fabulous country and you will have a great time, be a little careful of swimming in the lake Bilharzia is common long it's shores
Sound advise has been given about taking proper malaria precautions, although these are expensive to buy in the UK for this period of time it is worth it, if you were going for longer I'd suggest a starter course in the UK and buying the rest in country were they are far cheaper.
the usual vaccinations to consider are
Hepatitis A + B
Rabies although a serious condition is unlikely to be contracted unless you go near dogs or other animals, the jabs for rabies do not give protection as such, but give you more time to get to treatment, if you consider that you will likely be coming into contact, and by that I mean touching, animals then you might want to consider it, if your not likely to come in contact then don't worry, remember you don't pet dogs here unless you know the owners..
Pack good insect repellent against mosquitoes and midges, if you are going through Lilongwe you can buy them there, Peaceful Sleep I find the best against mosquitoes.
April is the end of the rains and from there on mosquitoes and midges will hatch, you will need a long sleeved shirt and trousers if by the lake. It will start to get chilly at night too as winter here will be approaching, a fleece is a good idea to have with you just in case, after 6 weeks you will acclimatise some and the cooler nights will start to feel colder to you, they start off balmy but you soon find them getting cold.
Transits in yellow fever areas such as Kenya, where you do not have your passport stamped seldom require a yellow fever certificate although if you plan to enter Zambia and then go on to South Africa you will,need one now.
If you plan to go to the high plateau beware you can suffer altitude sickness, not everyone gets it, but just make yourself aware of the signs so that you know. The northern area of Malawi suffers from some "bandit" activity, generally this is low key but be aware and don't hitch or travel alone if you can help it, saying that Malawi is generally a safe country the people are very welcoming, take the usual travellers precautions and don't flaunt expensive things, it applies everywhere not only here.
Have a great trip.
Live and work Northern Zambia