No, yeast infections are not contagious in the way a cold or the flu is. Yeast infections are caused by a fungus (more specifically, a yeast) called candida.

Candida is a commensal organism, meaning that under healthy conditions it lives harmlessly in our bodies. We all have candida living on our skin and in our intestines, and healthy women carry candida in their vaginas.

Men are also able to develop yeast infections on the penis.

When the body’s defenses are in balance, candida is not noticeable or bothersome. However, certain conditions can give candida the opportunity to grow excessively:

• A low-functioning immune system.
• Use of topical or oral corticosteroids such as prednisone (often for asthma or autoimmune conditions).
• Damage to the mucous membranes (the skin lining the mouth, vagina, and GI tract).
• Antibiotic use.

In these situations, candida can grow on the skin or in the lining of the mouth, GI tract, or vagina. The fungus creates a white patch or plaque across the mucous membranes. In the mouth, visible candida infection is called thrush. In the vagina, it’s a yeast infection. In people with weakened
immune systems, candida can grow almost anywhere in the body.

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Because candida is normally present in all our bodies, someone with thrush can’t exactly pass it to someone else — that person already has candida. To develop thrush, a yeast infection, or another candida infection, that person may have one or more of the above risk factors or conditions.

It is not yet clear if bacterial vaginosis and yeast infections are transmitted from one partner to another. All kinds of bacteria are exchanged during sex play. It may be that the bacteria exchanged between partners, especially new partners, may cause an overgrowth of the bacteria that cause bacterial vaginosis or yeast infections in the vagina.

Vaginitis caused by irritants, allergies, or lack of estrogen are not passed during sex.

Sexually transmitted diseases exist that carry similar similar symptoms to yeast infections, such as vaginitis or bacterial vaginosis. If you have never had a yeast infection and are unsure, please see a licensed physician for an official diagnosis. Only they can give proper treatment and cure your discomfort. At any time, condoms are the most effective way to avoid transmitting disease to a partner.

Source:

http://answers.webmd.com/answers/1195035/are-yeast-infections-contagious

http://www.plannedparenthood.org/health-topics/womens-health/yeast-infection-19854.htm

http://www.virtua.org/adam/health%20illustrated%20encyclopedia/2/17141.htm