Tetanus is caused by Clostridium tetani, an anaerobic, gram positive bacillus that produces a potent neurotoxin. In unimmunized or incompletely immunized individuals, infection follows contamination of a wound by soil containing clostridial spores from animal manure. The toxin reaches the CNS by retrograde axon transport, is bound to cerebral gangliosides, and appears to increase reflex excitability in neurons of the spinal cord by blocking function of inhibitory synapses. Intense muscle spasms result. Two thirds of cases in the United States follow minor puncture wounds of the hands or feet. In many cases, no history of a wound can be obtained. Injecting substances and drug abuse may be risk factors (in individuals who are not tetanus-immune). In the newborn, usually in underdeveloped countries, infection generally results from contamination of the umbilical cord. The incubation period typically is 414 days but may be longer. In the United States, cases in young children are due to failure to immunize. Eighty-five percent of cases occur in adults older than 25 years.