Your risk of contracting a disease or falling ill is the same as on the Scottish mainland, and tap water is potable everywhere. Many walkers like to refill their bottles from streams out on the hills. If you do so, there are general principles you should apply: never drink from water below human habitation, and never from still lochans, only fast-running burns. If you’re not confident, consider adding a chlorine tablet to the bottle, as you might elsewhere in the world.
One animal you should be mindful of is the tick, which is attracted to human blood and lurks in wooded, bushy and moorland areas. Ticks should ideally be removed as soon as possible, as leaving them on the body increases the chance of infection. They should be removed with special tick tweezers that can be bought in good travel shops. Failing that, you can use your fingernails: grasp the tick as close to your body as possible and pull steadily and firmly away at right angles to your skin.
One nasty surprise ticks appear to be increasingly transmitting is Lyme Disease, a bacterial infection often characterised by a circular rash and/or flu-like symptoms, and muscle and joint pain. The incidence of Lyme Disease is on the rise across the British Isles, including the Outer Hebrides. The NHS provides good information.
Unfortunately, the pesky wee fellas are here, usually from late May until September. That said, they are not as ubiquitous or bothersome as they are in Argyll or the central Highlands. If you sit in an open place for any length of time then they will certainly descend upon you, but in this part of the world a breeze of some kind or another usually comes along soon enough to keep them bearable.
You will rarely need recourse to a head-net of the kind that is so commonly used by walkers on Skye. It may or may not be comforting to know that while at least 35 species of midges have been identified in Scotland, only five species, and only the females, actually bite; that they are at their worst after nightfall or when there is cloud cover; and that they dislike bright sunlight. Repellents of varying effectiveness can be bought from your local chemist or outdoors shop.
The Outer Hebrides is an extremely safe place to visit. Violent crime is very rare indeed and in 2013–14 the police recorded precisely one instance of housebreaking. Bigger hazards are likely to be overestimating your driving capabilities on some of the narrow roads or avoiding sheep or deer that dart in front of your vehicle.
Most activities on the islands involve the great outdoors, so you should be mindful of the volatile weather and the often sporadic signal coverage for mobile phones. For walkers and climbers, sheer and exposed cliffs have a habit of springing themselves on you, particularly around Ness, Uig, Harris and South Uist, where they can rise up abruptly from sea level with startling contours. You should also take care walking on peat bog, which is far from a constant surface and can conceal eroded channels and buried river courses.
You should always ask locally about which beaches are safe for swimming. If kayaking, strongly consider going with a guide. The coastguard says that 86% of callouts on the islands involve visitors.
You will occasionally see signs for quicksand on beaches, including those at Eoropie on Lewis, Scarista on Harris and at Tràigh Tuath on Barra. The risk from quicksand is not of disappearing into the earth in the style of horror films but of drowning as the tide comes in. The Stornoway coastguard advises that you apply common sense: if you feel sand shifting under your feet, walk swiftly to firmer sand. If you encounter difficulty, call the coastguard on 999 immediately.