Taman Negara is Malaysia's largest national park, located on 4,343km² of primary rainforest with its own mountain range that features the Peninsular's highest summit, Gunung Tahan (2,187m). Sharing its borders with three different states (Pahang, Kelantan and Terengganu), the most popular entryway is through Kuala Tahan in Pahang, a small village across the Tembeling River from the park headquarters, closest to Jerantut town (67km away). An immensely popular nature destination, Taman Negara attracts visitors from all over the world, served by a good range of accommodation including budget hotels at Kuala Tahan and a boutique jungle lodge (Mutiara Resort) within the park.
While Taman Negara may conjure up images of a remote, ancient rainforest virtually untouched by civilisation, the park is actually well equipped with modern facilities that cater for tourists, including marked jungle trails (some with boardwalks), wildlife observation hides (some with bunk), a water taxi service (travel along the river), visitor office with interpretive center, and wide range of budget accommodation at Kuala Tahan (which also has restaurants, sundry shops and a clinic).
Most of the trails can be done on your own (if reasonably fit), though it's better to hire a guide if you're unaccustomed to jungle conditions and safety precautions. Malaria is of very little concern, unless you plan to spend months deep inside the rainforest. Dengue fever is also not a worry, as the Aedes mosquito prefers disturbed habitats near lots of people (eg big villages and urban areas). But watch out for bees and wasps for their obviously painful sting.
The rainforest is a hot and incredibly humid environment (wear thick clothing to your own peril) in day, but freezing cold after midnight. Heavy rainfall is very common at the last quarter of the year, stretching for a few months into the next. Rain can last for a few hours or more, but it is more common to encounter short thunderstorms - still enough to turn most trails into a mud bath. Leeches are abundant further inside the rainforest; prepare salt or suitable insect repellant if you don't want to get your socks caked in blood.
While encounters with large wild animals such as elephants, gaur and tigers are rare (if it does happen, ensure a very safe distance), it's much easier to meet semi-tame tapirs, deer and wild boar that frequent the park HQ grounds for easy food from visitors. Squirrels and jungle rodents are also untimid and will approach visitors or raid bags with food, especially those around wildlife hides.