There is no clear boundary concerning how deep a wound needs to be to carry a risk of tetanus; mainly, it depends on whether the conditions allow for anaerobic bacteria to thrive. Particularly, wounds that are very small on the surface but deep can lead to substantial proliferation of tetanus bacilli. A common example is a foot wound caused by stepping on a nail, where the exterior opening of the wound is small, but it is deep inside, and the nail surface may be rusty and also carry a large amount of tetanus bacilli. If the wound is not promptly and thoroughly cleaned, it can lead to massive growth of tetanus bacilli, resulting in the onset of severe illness in the patient.