Not all rusty nails cause tetanus. Tetanus is primarily associated with the bacteria Clostridium tetani, commonly abundant in soil, dust, and animal feces. While rusty nails can provide a route for the bacteria to enter the body through a puncture wound, the presence of rust itself does not directly cause the infection.

To develop tetanus, several conditions must be met:

1. Introduction of bacteria: The bacteria must be introduced into the body through a puncture wound or open injury. Rusty nails, if contaminated with C. tetani spores, can facilitate this introduction.

2. Lack of oxygen (anaerobic environment): C. tetani thrives in environments where oxygen is limited. Once inside the body, the bacteria can multiply and produce toxins in this anaerobic environment.

3. Compromised immune system: A healthy immune system usually effectively combats the bacteria. However, individuals with compromised immune systems may be more susceptible to developing tetanus.

4. Non-sterile wound: Proper cleaning and wound care can reduce the risk of infection. If a wound caused by a rusty nail is not adequately cleaned and dressed, it provides a suitable environment for bacterial growth.

Therefore, stepping on a rusty nail does not guarantee tetanus. Many cases are unrelated to rusty nails, and tetanus infections can be prevented through appropriate wound care, immunization, and timely treatment. Tetanus shots, which contain antibodies to counteract the bacteria's toxins, play a crucial role in protecting individuals from this infection.