The Unseen Guest: Parasites and the Human Condition Before 1908

Long before the twentieth century, physicians understood the human body as a place that could be inhabited by other living creatures. Worms were not regarded merely as objects in the intestines. They were described as active inhabitants capable of disturbing digestion, appetite, circulation, movement, sensation, temperament and thought.

Nicolas Andry’s 1701 An Account of the Breeding of Worms in Human Bodies described their “effects, symptoms, and prognostics,” together with methods of avoiding and removing them. Worms were considered in connection with the bowels, blood, organs, nerves and general condition of the patient. Read Andry’s 1701 work.

Valeriano Luigi Brera continued this tradition in A Treatise on Verminous Diseases, translated into English in 1817. Brera arranged the subject around the natural history of intestinal worms, their origin in the body and the constitutional illnesses attributed to them. The expression “verminous disease” reflected the belief that parasitic illness could become a condition of the whole person rather than remain confined to the gut. Read Brera’s 1817 treatise.

From punishment to natural history

Parasites once carried religious and moral meanings. Worms appeared as symbols of corruption, decay and divine punishment. By the nineteenth century, professional parasitologists increasingly described them as living creatures with their own development, reproductive cycles and means of entering a host.

In his 1879 book Parasites, physician and helminthologist T. Spencer Cobbold rejected the belief that internal worms were punishments for wrongdoing. He argued that such ideas interfered with the natural study of parasitic life.

Cobbold described how infection could arise through meat, water, vegetables, animals, soil and sewage. Tapeworm eggs could pass from human waste into the environment and return through contaminated food or water. The condition of an individual body was therefore connected to sanitation, agriculture, food preparation and the surrounding community. Read Cobbold’s 1879 Parasites.

When parasites affected the mind

Nineteenth-century parasitologists wrote directly about changes in thought and behaviour. Cobbold collected reports of cysts in the brain associated with headaches, convulsions, epilepsy and mental disturbance. He noted that these disturbances could occur with or without epileptic attacks.

Rudolf Leuckart’s 1886 textbook, The Parasites of Man, and the Diseases Which Proceed from Them, provided a more extensive account. Where cystic forms of tapeworm occupied the brain, Leuckart listed headache, fatigue, lethargy, giddiness, cramps, paralysis, epilepsy and psychical disturbance. The mental conditions described in the cases included what physicians then called melancholia and mania.

Leuckart also reported nervous symptoms in people carrying intestinal tapeworms. His list included ringing in the ears, hallucinations, giddiness, fainting, epilepsy, involuntary movement and mental disease. He wrote that some conditions disappeared after removal of the worm, although he also warned that such symptoms were not sufficient to diagnose a parasite without physical evidence.

For Leuckart, a parasite’s effects depended upon its species, number, location, movement and relationship with its host. It might consume nourishment, damage tissue, obstruct an organ, disturb the blood or press upon the brain. Read Leuckart’s 1886 textbook.

Parasites in the blood

The discovery of blood parasites widened the subject beyond intestinal worms. In an 1887 address, physician William Osler described microscopic organisms found in people suffering from malaria.

Osler connected these organisms with the destruction of red blood corpuscles, pigment formation, enlargement of the spleen, anaemia and recurring fever. In grave cases, physicians observed comatose states. The microscope allowed them to associate exhaustion, pallor, fever and collapse with organisms visible in the blood. Read Osler’s 1887 address.

The “germ of laziness”

In 1903, American publications reported zoologist Charles Wardell Stiles’s investigations of hookworm disease. The popular press called it the “germ of laziness.”

A report in Popular Science Monthly described “extreme lassitude, both mental and physical” as an important symptom. The explanation offered was that blood impoverished by the parasite failed to nourish the brain and muscles properly.

Hookworm was connected with weakness, reduced willingness to exert oneself, interrupted schooling and delayed physical and mental development. Conduct previously described as laziness or lack of character was presented as a bodily condition. Poverty, low productivity and limited education were discussed as possible consequences of widespread infection. Read the 1903 account.

Sleeping sickness and personality

The Royal Society’s 1903 reports on sleeping sickness described another connection between parasites and human behaviour. Investigators reported that the earliest sign might be a slight change in the patient’s former mental attitude.

Relatives might notice dullness, indifference, slowed movement and disinclination to work. Increasing lethargy followed, accompanied by reduced eating, tremors and withdrawal from the surrounding world. In the final stages, the patient entered profound sleep or coma.

Investigators examined trypanosomes in blood and cerebrospinal fluid while recording individual case histories. Alterations of disposition, attention, memory and consciousness became part of the clinical description of parasitic illness. Read the Royal Society’s 1903 reports.

The human being as a host

By the opening years of the twentieth century, professional literature presented parasites as capable of influencing nourishment, blood, muscular strength, sensation, sleep, intellectual development, emotional disposition and consciousness.

Their effects also extended beyond the individual. Physicians and zoologists wrote about lost labour, interrupted education, declining households, contaminated food, diseased animals and weakened communities. Parasitism connected the internal condition of the body with water, soil, housing, agriculture, sewage and the movement of animals and people.

In these pre-1908 publications, strength of body, clarity of mind and capacity for work were all described as vulnerable to the unseen lives occupying the human host.

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