Usually, transmission from one person to another is insignificant, as the fungus lives on human skin anyway. During a bad infection, however, the fungi can be passed on during sex and can lead to an infection for your partner as well. That usually occurs during unprotected sex between a man and a woman.
People with HIV and diabetes, people with cancer who are undergoing radiotherapy or chemotherapy and pregnant women are particularly susceptible. You may also be susceptible if you have recently been taking antibiotics or cortisone treatments, as these can kill the bacteria on the mucous membranes which usually control the fungus.