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Lyme disease is a tick-borne bacterial infection caused by members of the Borrelia burgdorferi species complex and transmitted by infected Ixodes ticks. Early illness can include fever, fatigue and an expanding rash called erythema migrans; later involvement can affect joints, the nervous system or the heart. The rash has varied appearances and need not form a textbook bullseye.^[ref]

The bacterial link emerged from a specific line of evidence. In 1982, Willy Burgdorfer and colleagues reported spirochaetes in ticks from an affected area, cultivation of the organisms, and reactions with antibodies from patients with Lyme disease. This connected a vector-associated organism to the human illness, rather than inferring a cause from an association with outdoor exposure alone.^[ref]

Willy Burgdorfer investigated tick-borne infectious diseases. In 1982 he and collaborators reported a spirochaete isolated from ticks associated with Lyme disease. Infected ticks produced persistent skin lesions in rabbits, and patients' blood serum contained antibodies reacting with the organism in indirect immunofluorescence tests.^[ref]

These were complementary observations: finding an organism in a suspected vector, observing an effect after feeding, and detecting a human immune response each supplied a different link in the causal argument. The original report cautiously proposed involvement in Lyme disease; it did not rest its conclusion solely on resemblance under a microscope.