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Medically important protozoa include Plasmodium, the cause of malaria, Giardia and Cryptosporidium, which can cause gastroenteritis, and Trichomonas, which is a sexually transmitted cause of vaginitis.

The most common routes of infection with protozoa are by consuming them in food or water or via an insect vector such as a mosquito (Goering et al. 2007). Cross-infection in the course of healthcare is uncommon but not unknown.

Helminths

‘Helminth’ is a generic term for parasitic worms. A number of worms from three different groups affect humans: tapeworms, roundworms (nematodes) and flukes. Transmission is generally by ingestion of eggs or larvae, or of infected animals or fish, but some are transmitted via an insect vector and some, notably the nematode Strongyloides, have a larval stage that is capable of penetrating the skin.

Helminth infections can affect almost every part of the body, and the effects can be severe. For example, the Ascaris worm can cause bowel obstruction if there are large numbers present; Brugia and Wuchereria obstruct the lymphatic system and eventually cause elephantiasis as a result; and infection with Toxocara (often after contact with dog faeces) can result in epilepsy or blindness. However, cross-infection in healthcare is not normally considered a significant risk.

Arthropods

Arthropods (insects) are most significant in infectious disease in terms of their function as vectors of many viral, bacterial, protozoan and helminth-caused diseases. Some flies lay eggs in the skin of mammals, including humans, and the larvae feed and develop in the skin before pupating into the adult form, and some, such as lice and mites, are associated with humans for the whole of their life cycle. Such arthropod infestations can be uncomfortable, and there is often significant social stigma attached to them, possibly because the creatures are often visible to the naked eye. The activity of the insects and the presence of their saliva and faeces can result in quite severe skin conditions that are then vulnerable to secondary fungal or bacterial infection (Goering et al. 2007).

Lice

Species of Pediculus infest the hair and body of humans, feeding by sucking blood from their host. The adult animal is around 3 mm long and wingless, moving by means of claws. It cannot jump or fly, and dies within 24 hours if away from its host, so cross-infection is normally by direct contact or transfer of eggs or adults through sharing personal items.

Scabies

Scabies is caused by the mite Sarcoptes scabiei, an insect less than 1 mm long, which burrows into the top layers of skin. Infestation usually starts around the wrists and in between the fingers because acquisition is normally by close contact with an infected individual. The female mites lay eggs in these burrows and the offspring can spread to other areas of skin elsewhere on the body. The burrows are visible as a characteristic rash in the areas affected. In immunocompromised hosts or in those unable to practise normal levels of personal hygiene, very high levels of infestation can occur, often with thickening of the skin and the formation of thick crusts. This is known as ‘Norwegian scabies’ and is associated with a much higher risk of cross-infection than the normal presentation. Scabies is most often associated with long-stay care settings, but there have been reported outbreaks associated with more acute healthcare facilities.

Prions

Prions are thought to be the causative agents of a group of diseases called transmissible spongiform encephalopathies (TSEs), the most well known of which are Creuzfeldt–Jacob disease (CJD) and its variant (vCJD), which has been associated with the bovine spongiform encephalopathy (BSE) outbreak in Great Britain in the late 1980s and early 1990s. TSEs cause serious, irreversible damage to the central nervous system and are fatal. They are characterized by ‘plaques’ in the brain that are surrounded by holes that give the appearance