Folliculitis: This is an infection of the hair follicles, often caused by Staphylococcus aureus. It's a common condition that can occur on any part of the body with hair, including the scalp, making it the most likely diagnosis for a scalp infection due to its prevalence and the typical presentation of red, itchy, and sometimes pus-filled bumps.

Impetigo: A highly contagious skin infection that can affect the scalp, typically caused by Staphylococcus aureus or Streptococcus pyogenes. It presents as red sores on the skin that can burst and form light brown crusts.
Ringworm of the Scalp (Tinea Capitis): A fungal infection that affects the hair shafts and follicles of the scalp, leading to hair loss, itching, and redness. It's more common in children but can occur in adults as well.
Seborrheic Dermatitis: While not typically considered an infection, it can be complicated by secondary infections. It causes scaly patches, red skin, and stubborn dandruff due to an oily scalp.

Meningitis: Although not directly a scalp infection, meningitis can present with symptoms that might initially be mistaken for a scalp infection, such as fever, headache, and neck stiffness. It's crucial to consider this diagnosis, especially if there are signs of severe illness or neurological deficits.
Osteomyelitis of the Skull: An infection of the bone, which can occur in the skull and might be related to a scalp infection, especially if there's a history of trauma or surgery. It requires prompt diagnosis and treatment to prevent serious complications.
Cellulitis: A bacterial skin infection that can spread rapidly and become serious. While it can occur anywhere, when it affects the scalp, it can be particularly concerning due to the rich blood supply and potential for spread to other areas.

Kerion: A severe form of tinea capitis that leads to an inflammatory, painful mass on the scalp. It's less common than other forms of ringworm but can be a complication of untreated or severe fungal infections.
Actinomycosis: A rare chronic bacterial infection that can affect the scalp among other areas, causing abscesses, sinus tracts, and scarring. It's often associated with poor dental hygiene or oral surgery.
Sporotrichosis: A fungal infection caused by Sporothrix schenckii, typically acquired through skin prick injury from a thorn or other vegetation. It can cause lesions on the scalp, among other areas, and is more common in gardeners or those who work with plants.