Getting around Tonga will generally be fine. There have been some blips on the country's maritime safety track record, with the sinking of ferry Princess Ashika ceasing all inter-island travel temporarily in August 2009. In late 2010, a new ferry, called the MV 'Anga'ofa Otu, debuted and resumed service between islands. The ferry does meet international safety standards. You can also opt for ferry service operated by local companies or individuals, but they may not be as safe as they don't always obey regulations.
Because it is an island nation in the Pacific, Tonga is prone to several forms of natural disaster. It does sit on an active earthquake zone, but normally only ever experiences minor tremors. The earthquake zone does mean tsunamis are more frequent here. In September 2009, these deadly forces met when an earthquake near Samoa triggered a tsunami that made landfall on Tonga, resulting in nine deaths. Cyclones also can happen in Tonga, with the season usually running November to April, though these weather events can happen in other months, too. During these storms, flooding and severe winds can occur, and power and transportation service may cease.
Even if you are a strong swimmer, do not get too confident on the beaches of Tonga. There are coral reefs that are particularly sharp in Tongatapu and PangaiMotu. Strong rip currents can also endanger even the best swimmers. Several travelers have died before due to the strong currents. Certain spots are more hazardous than others, and it is advisable to seek out locals to find out where these locations are. The Indo-Pacific 'Man o' War' is present in the waters around Tonga and may give you a sharp sting.
Sea snakes also swim about, but are unlikely to attack. Poisonous cone shells near coral reefs can sting you and, in severe cases, lead to fainting and trouble breathing. On land, there are other critters than can string you, like the Molokai, or centipede, which like to crawl into shoes. Wild dogs roam the islands, and are usually safe when alone, but can be aggressive in packs. Lonely Planet advises pretending to throw a stone at the pack to distract them.
In terms of illness, there isn't much beyond the normal list of island sicknesses in Tonga. Dengue fever can occur if bitten by a mosquito, and ciguatera is a random poisoning the occurs in normally clean, safe fish. It happens most often in meat-eating reef fish like red snapper and Spanish mackerel, so you may want to avoid those types of fish on the menu. Lonely Planet points out that the water in Pago Pago, Apia and Nuku'alofa is generally safe to drink, but elsewhere, you should boil it first. Use caution when eating anything with produce like salads, and if preparing your own food, peel all fruits and vegetables before eating. Stick to food that is hot to avoid contamination.
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