The rainy season on the west coast of Peninsula Malaysia typically comes between April and October or November – though the levels of rainfall are only marginally higher during these months, and often the rains don't really arrive until June or July. Even during the monsoon, the rains tend to come in sharp bursts, usually during the afternoon, with sunshine for the rest of the day. Penang, however, does get heavy rains in September and October.
The east coast monsoon, which falls between mid-November and February, is more marked, churning up the seas and making some of the island destinations inaccessible. These rains also affect the interior of Peninsula Malaysia, and the whole of Malaysian Borneo.
Malaysia is surprisingly free of health concerns, with drinkable tap water in most places and low rates of tropical disease. Officially, no inoculations are required, though it is wise to update your tetanus, hepatitis A, polio and typhoid inoculations before travelling. Malaria is a moderate problem in some remote areas, and in the interior of Malaysian Borneo. In developed parts of the country (including virtually the whole of Peninsula Malaysia) malaria has been wiped out. Doctors are likely to advise those travelling to Malaysian Borneo to take malaria tablets.
At certain times of year, jellyfish can be a problem off the beaches. Sea urchins, too, are prevalent, so watch where you stand.