Sinus infections are caused by microorganisms proliferating in blocked sinuses. The offending microbes may be bacteria, viruses, or, rarely, fungi, and treatment for sinusitis will vary accordingly. It is usual to wait viral infections out, but antibiotics will probably be used against bacterial sinusitis that has gone on for 10 days or more. Fungal infections will need more rigorous treatment, sometimes requiring endoscopic surgery in addition to anti-fungal medicine.
So, is sinus infection contagious?
The answer, simply, is "no".
It is more likely than not that the microorganisms that cause sinusitis were already present in the nasal cavity or sinuses even before the infection became full-blown instead of being caught from another person. Being kept in the sinuses by blockages resulting from swellings of the mucus membrane simply gave them the opportunity to grow and multiply and make a body painfully aware of their presence.
Perhaps the question shouldn’t be "Is sinus infection contagious?" but rather "Is the condition that can lead to sinus infection contagious?" The blockages that allow sinus infections to occur are sometimes the result of the common cold, the flu, or other ailments associated with a clogged nose which can easily be passed from one person to another. Should you be having trouble with a clogged nose and suspect sinusitis, consult your physician.