(aye aye) Infective( viral, bacterial, fungal) or Inflammatory eg psoriasis or a pustular drug reaction. Common causes include Staph folliculitis , modified fungal infection or if the vesicles are grouped herpes simplex. Pustules on the face are Acne, Rosacea, Staph folliculitis or H Simplex if grouped.
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When we see pustules we have a tendency to think infection and often limit ourselves to only bacterial infections at that. Remember pustules can occur with fungal and viral infections as well and never forget that some pustules are not due to infection but to infiltration of the skin by neutrophils in Inflammatory skin diseases such as psoriasis and drug eruptions (localised AGEP and halogenodermas) and Sweet's syndrome.
There are also a few other very rare inflammatory disorders such as acrodermatitis enteropathica (zinc deficiency) and the glucagonoma syndrome from underlying pancreatic malignancy that can have pustules at the advancing edge of the lesion and thereby simulate a fungal infection.
Other rarer inflammatory conditions with pustules in an advancing edge are subcorneal pustular dermatosis and IgA pemphigus, erosive pustular dermatosis and folliculitis decalvans of the scalp. Some pustules are due to eosinophils such as those in neonates with Toxic erythema of the newborn and neutrophils in transient neonatal pustulosis of the newborn.
Infantile acropustulosis of the feet is also non infectious and probably a reaction to previous scabies. Behcet's can give inflammatory pustules at sites of injury (pathergy) and you can see them in Bowel associated dermatitis -arthritis syndrome.
Eosinophilic pustular folliculitis is seen in HIV cases.
Pustules can also bee seen in Septic vasculitis. So remember Inflammatory as well as Infectious!
The images below include variants of pustular psoriasis, both localised and generalised and infected pompholyx eczema.
