As with its neighboring countries malaria can be a problem. The lake is freshwater and is prone to bilharzia, especially in the Cape Maclear area. Symptoms of bilharzia can take months to surface, if you think you've been exposed to it you can get a very cheap pill from the local pharmacists that will kill it before it even shows its face. It's a good idea to take care of it before leaving Malawi as it will be much more expensive back home.
The adult HIV prevalence in the country is at 14% or 1 in 7 adults. Do not have unprotected sex. Do not use injecting drugs.
The official languages of Malawi are English and Chichewa. English is widely spoken in urban areas and by the well-educated upper class, though outside of that, a few words in Chichewa will go a long way. Chichewa is the first language of the majority of the population, and knowing Chichewa will get you by in most of Malawi though in some very remote areas, learning the local tongue might be essential. Locals always appreciate any attempts by foreigners to speak Chichewa and learning at least a few basic greetings would do well to ingratiate yourself to the locals. Tumbuka is the first language for many people in the north of the country. Chiyao is spoken by the Yao people who live mostly in the Southern District of the country. A multi-cultural country, Malawi has over a dozen indigenous ethnic groups, each with its own distinct language. However, even in those areas, many younger people will be bilingual in the local language and Chichewa.
Malawi has both patriarchal and matriarchal ethnicities and cultures. In the cities, men tend to be more respected than women, but the reverse might be true in the rural villages depending on ethnicity. Whites tend to be well-respected, a holdover from colonial times, but this is largely a Malawian's way of being courteous. Accept their hospitality. They are an exceptionally friendly people.
Malawians, especially those from very rural areas where they don't see many whites, can be quite curious when they do come upon a white traveler. To a Western mindset, this might be interpreted as unnecessarily staring at you or talking about you in front of you. Be prepared to be greeted by kids yelling mzungu, mzungu! and to answer lots of questions about yourself. Even relatively mundane items like mechanical pencils can draw a crowd of onlookers.
Malawians are in general extremely courteous, and a part of that courtesy is shaking hands, speaking softly, and referring to travelers and others with respect. Malawians avoid rudeness. It is common for Malawi men to hold hands when they've gathered together to chat, and this shouldn't be given a sexual interpretation when it is encountered.
Culturally, women should not wear shorts or mini-skirts, especially when traveling outside the lodge/camp. A woman in shorts or a short skirt is considered to be provocative, as well as rude. Many female visitors wear wraps that are available in the stores and markets of major cities. These are generally made of bright, colored patterns and can be extremely attractive. Low-cut tops on women, while discouraged, are not nearly as provocative. Men in the cities tend to wear slacks and not shorts, as shorts are generally worn only by school-age children, so when a man wears shorts it can be viewed by Malawians as rather silly.
Finally, when meeting a Malawian — even to ask a question — you should always say hello and ask how they are. Properly greeting a Malawian is very important. They are uncomfortable with the Western notion of simply "getting to the point." Courtesy is a must, at all times, because not to be courteous is to show disrespect.