Recurrent nodules , abscesses and chronic suppurating lesions in the apocrine gland of the skin. It can vary from mild to severe:
" Most patients have a mild form of the disease, manifested as painful large and deep- seated nodules. These lesions can resolve spontaneously, persist as “silent” nodules, or lead to abscess formation. In contrast, patients with severe HS have chronic, painful, suppurating lesions that persist for years. Chronic lesions usually involve multiple areas connected by inflamed and suppurating sinus tracts surrounded by hypertrophic scars (PubMed)."
Bad luck, no one knows the cause.
"Staphylococcus lugdunensis was cultured as a unique or predominant isolate from 58% of HS nodules and abscesses, and a polymicrobial anaerobic microflora comprising strict anaerobes, milleri group streptococci, and actinomycetes was found in 24% of abscesses or nodules and in 87% of chronic suppurating lesions (PubMed)."
With that microbiology probably worth careful cultures.
A study using molecular methods found "Bacterial cultures detected anaerobes in 83% of lesions vs 53% of control samples, combined with milleri group strep- tococci and actinomycetes in 33% and 26% of cases, respectively. High-throughput sequencing identifed 43 taxa associated with HS lesions. Two gram-negative anaerobic rod taxa, Prevotella and Porphyromonas, predominated, contrasting with a reduced abundance of aerobic commensals... Two main additional taxa, Fusobacterium and Parvimonas, correlated with the clinical severity of HS." (PubMed).