The bacterial form that can be curved like a spiral, thick and rigid, or thin and flexible is called a spirillum.

Here's a breakdown:

* Spirilla: These bacteria have a rigid, spiral shape. They are typically thicker and more rigid than spirochetes.

* Spirochetes: These bacteria have a flexible, spiral shape. They are thinner and more flexible than spirilla.

Here are some examples:

* Spirilla: *Campylobacter jejuni* (causes food poisoning)

* Spirochetes: *Treponema pallidum* (causes syphilis) and *Borrelia burgdorferi* (causes Lyme disease).