Pustule
A pustule is a small raised lesion containing pus, a mixture of immune cells and fluid. Despite the association, its presence does not establish that an infection is responsible: several non-infectious conditions produce pustules.
A raised lesion with visible pus in it. The description is straightforward; the inference people draw from it is where the trouble starts.
Pus is a collection of immune cells, chiefly neutrophils, together with fluid and cellular debris. Its presence means the immune system has responded vigorously at that spot. It does not, by itself, mean an organism caused the response.
That distinction matters clinically and it is routinely missed. Acne produces pustules through inflammation within a blocked follicle rather than through an infection needing to be eliminated. Some forms of psoriasis are described as pustular, and they are not infections either. Assuming that visible pus calls for something aimed at bacteria is a common and unhelpful leap.
Within the descriptive vocabulary a pustule sits beside a papule, which is raised and solid, and a vesicle, which is raised and contains clear fluid rather than pus. The three differ in content, and what the content is tells a clinician something about the process underneath.
Distribution and what accompanies them then carry the rest. Pustules confined to follicles suggest a different set of possibilities from pustules unrelated to them, which is the sort of observation an examination is for.
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