The individual species are not equally frequent everywhere. Some are mostly found inside the house while others are mostly found outside.

These are mainly troublesome indoors: Head lice, body lice, pubic lice, bedbugs, the masked hunter, larder beetle larvae, stable flies, human fleas, cat fleas, dog fleas, brown dog tick, pigeon mites, mites on birds, house dust mites, storage mites and spiders.

These are mainly troublesome outdoors: Thrips, common flower bugs, water bugs, rusty tussock moth larvae, mosquitoes, black flies, midges, horse flies, bees, hornets, ants, hedgehog fleas, bird fleas, caster bean tick, harvest mites, stinging jellyfish, leeches, greater weevers and the European adder.

Of course, the line that separates the two groups in this division of the animals is quite thin. The outdoor species may stray into houses or they can be brought inside. Mosquitoes bite people inside the house while other animals, such as horse flies, do not bite while indoor.

From birds’ nests, the following animals may bite: lyctocoris campestris, Martin bug, louse flies, bird fleas, mites on birds and pigeon mites. Cat fleas, itch mites and fur mites can attack a dog or cat. By bathing in stagnant water you may experience close encounters with the common backswimmer, the saucer bug and leeches. In sea water the biggest troublemaker is probably the stinging jellyfish. Greater weever stings may be experienced on the sandy bottom of the ocean.

There are also geographical differences. In Denmark, the harvest mite is mostly seen in Thisted and environs. The Eastern Pine Processionary with its etching hair is known from Dueodde on the island of Bornholm. The medicinal leech lives on Bornholm and in North Zealand. Black flies can form very large populations at the lower reaches of Gudenåens and Susåen. After traveling in southern Europe, bedbugs, house centipedes, scorpions, etc. can be brought home in luggage or camping equipment. Dogs that come to Denmark from the Mediterranean can bring back brown dog ticks.

All over the world, itch mites and crab lice can be the result of sharing a bed with another person.