This is a very good question that you ask about whether a sinus infection is contagious. There are a couple of things that I must define before being able to accurately give you an answer. First of all, most otolaryngology (aka ENT or Ears Nose Throat) physicians will agree on the terminology for something to truly be called a sinus infection, you must have symptoms (like obstruction, drainage, etc) for at least 2 weeks. The sinonasal cavity is lined with mucosa and erectile tissue. This tissue has the ability to swell, and can obstruct the normal outflow of the sinuses. This in and of itself doesn't necessarily mean that you have a sinus infection that requires antibiotics. In fact, more often than not the mucosa has become swoolen because of a viral URI (upper respiratory tract infection) like a "cold". There are many other causes of mucosal inflammation and swelling (like allergies), but viruses cause the majority of acute nasal obstructive symptoms. If the normal sinonasal mucous is obstructed for long enough, one can develop a bacterial superinfection on top of the viral infection, and this is when antibiotics are necessary. Most of the time the bacteria that cause the infection are bacteria that are normally found within the nose. So to answer your question about being contagious, it depends on what you are refering to...viruses are quite contagious, but how it may affect the next person can be variable (meaning they might not get sinus congestion if infected). Typically the cases of bacterial sinusitis are not considered dangerously contagious when compared to other illnesses. Nonetheless the best prevention from getting ill, or passing on an illness
is thorough and regular hand washing. If you have more questions about sinusitis, I would recommend that you make an appointment with an ENT to get evaluated and discuss your concerns. Best of luck.