Tetanus is a condition caused by a nerve toxin that is produced by the bacterium Clostridium tetani, a cousin of the bacteria that cause gangrene and botulism. It remains a serious worldwide public health problem, killing over 500,000 people each year.
In developed countries, however, the introduction of a vaccine in the 1950s has all but eradicated this disease. The cases of tetanus that do occur are often seen in those who either have not been vaccinated or who have failed to keep their immunizations up-to-date, as protection from the vaccine declines over time.
Clostridia are anaerobic bacteria, meaning they thrive best in the absence of oxygen. They're commonly found in soil, but can also be found in the lower intestines of mammals. They can produce spores that can survive for years.
Anyone who cuts himself or herself with a dirty object is at risk of getting Clostridium tetani in the wound. The bacteria are only really dangerous if they're in wound tissue that's cut off from a good oxygen supply. "Crush" wounds and deep puncture wounds are good candidates for such infections, as are burns, surgical wounds, and punctures with dirty needles. Most cases of tetanus in North America are found in older people following surgery and in intravenous drug users who reuse unclean syringes.
There's also the occasional case of tetanus linked to childbirth. The mother can develop infection in the uterus, and the baby in the stump of the umbilical cord.
As the bacteria grow and multiply, they produce a nerve toxin. As with botulism, it's the toxin and not the organisms themselves that does the damage. The toxin binds to nerve endings that normally serve to calm the muscles. In the absence of receiving such calming signals, the muscles contract and become rigid and very sensitive to external input, leading to spasms.