I'm very prone to sinus attacks, always on the right side. It's been my misfortune for the last three years to have suffered from frequent bouts of cellulitis, so I've been on big doses of antibiotics a lot (I'm now on a prophylactic low dose of Penicillin V for a whole year!). I can tell you that the antibiotics do not stave off, nor cure, sinus attacks. I suppose that means that they're caused by viruses.