A sty is contagious in the sense that the bacteria that cause it can be transmitted quite easily, both from one person to another and from one part of your body to another. However, in most normal, healthy human beings a sty is never contagious. If one family member develops a sty, it is extremely unlikely that another member of the family will also develop one. The bacteria that causes sties is known as Staphylococcus aureus, and is almost always present on our skin. It is only when they start to multiply at an unusually high rate, usually as a result of stress, poor hygiene, poor nutrition, or irritation from other substances, that a sty develops.
In spite of this, if you have a sty, it is still a good idea to avoid rubbing or otherwise touching your eyes every now and then. Although spreading the infection to another person is not very likely, there is still a chance that the bacteria could be transferred from your eye to your hands and then to another person's hands, and eventually to their eyes. If someone in your household is particularly susceptible to sties for some reason - low immunity, frequent blepharitis infections, and so on - this is all the more reason to be careful. In such a case it is also advisable that you do not share items such as pillows, towels, spectacles, and sunglasses, which are likely to have a lot of Staphylococcus aureus bacteria on them from the infected eyelid.
In addition, if you yourself tend to keep getting sties, it would be a good idea to be more careful with such objects by sterilizing them frequently. This will help ensure that the bacteria do not remain on one of these objects to subsequently cause another sty. Besides these precautions, you should also be more diligent about hygiene as far as your eyes are concerned. Wash your face regularly, and ensure that your eyelids are clean. If you use any kind of make up on or around your eyes, wash it off thoroughly as soon as you get back home. Keep your hands clean too, as they tend to pick up bacteria and can easily transfer them to your eyes. This is especially important if you use contact lenses. Disinfect the lenses thoroughly with the recommended frequency and using the recommended procedure. If in spite of this you frequently develop sties, you should consult a doctor.
answered by G M