Papules that are infected and contain fluid are known as:

Pustules are indeed defined as inflamed, elevated lesions that contain a mixture of bacteria, white blood cells, and serum. They are characterized by their appearance, often resembling small blisters with a yellowish or whitish center. This fluid-filled nature distinguishes pustules from other types of skin lesions. Their formation typically occurs in the context of acne or other skin infections where the accumulation of pus indicates a response to infection, making them a prominent feature in various dermatological conditions.

In contrast, blisters are primarily filled with clear liquid rather than pus and often form due to friction, burns, or other irritants. Wheals are raised, itchy areas that result from allergic reactions and do not generally contain pus, while nodules are larger, solid lesions that extend deeper into the skin and are not typically fluid-filled. Thus, pustules are specifically tied to infections, containing the characteristics that define their clinical presentation.