What causes blisters in the mouth? Viruses?

Several Causes. There are many different conditions which cause blisters in the mouth. The most common are caused by the herpes virus. These can be painful, unsightly, and contagious, and should be examined by a dentist. Less common conditions can be much more dangerous and harmful, so they should be ruled out. Differentiating is usually easy based simply on appearance and history. Your dentist can help you.
Many things. Blisters, erosions and sores in the mouth can be caused by many different things. Some are contagious (herpes simplex virus) and many are not (canker sores, lichen planus, pemphigus vulgaris). Mouthwashes can be very harsh on the sensitive mucosa in the mouth and can lead to ulcers/erosions. Have this checked out to find the exact cause as this is important for long term treatment decisions.