What causes recurring blisters on bottom of feet?

There are a lot of causes for plant (bottom of foot) blisters:

Friction due to bad-fitting shoes.

Socks that are not clean every day when you dress.

Shoes or socks that are wet a lot of the time.

Allergy to something in the socks or shoes.

A type of eczema.

Athlete's foot.

A rare type of Diabetes.

Burns.

Hand, foot, and mouth disease in a child.

You should consult a doctor, but you can try keeping the foot very, very clean, use Tinactin powder after every foot wetting, an anti-athlete's foot cream, like Tinactin or Lotramil, Aloe gel applied a couple of times per day.

When you go to the doctor, she will ask you a bunch of questions which help determine which kind of the above blisters the person has. She will focus on the most common type of foot blisters.