Lyme Disease: The presence of a target rash, also known as erythema migrans, is a hallmark symptom of Lyme disease, which is caused by the bacterium Borrelia burgdorferi and transmitted to humans through the bite of infected blacklegged ticks. The rash typically appears within a week or two after being bitten.

Southern Tick-Associated Rash Illness (STARI): This condition presents with a similar rash to Lyme disease but is caused by the bite of the lone star tick. It's less common than Lyme disease but should be considered, especially in regions where the lone star tick is prevalent.
Erythema Multiforme: A skin condition characterized by target-like lesions, often triggered by infections or medications. While it doesn't typically present as a single large rash like erythema migrans, it could be considered in the differential due to the targetoid appearance of the lesions.
Cellulitis: A bacterial skin infection that can cause redness, swelling, and warmth of the skin, sometimes resembling a target rash. However, cellulitis usually has distinct borders and may be accompanied by systemic symptoms like fever.

Rocky Mountain Spotted Fever (RMSF): Although less common, RMSF is a serious and potentially life-threatening disease caused by Rickettsia rickettsii, transmitted by the bite of an infected tick. Early symptoms can include a rash, fever, and headache, making it crucial not to miss this diagnosis due to its severity and the need for prompt treatment.
Southern Tick-Associated Rash Illness (STARI) with potential for co-infections: While STARI itself is not typically severe, the bite of a lone star tick can also transmit other pathogens, such as Ehrlichia or Anaplasma, which cause ehrlichiosis or anaplasmosis, respectively. These diseases can be severe and require specific treatments.

Morgellons Disease: A rare condition characterized by skin lesions and the presence of filaments or fibers under the skin. While its cause is controversial and not fully understood, it could be considered in the differential for a target-like rash, especially if accompanied by other systemic symptoms.
Weil's Disease (Leptospirosis): Caused by Leptospira bacteria, this disease can present with a wide range of symptoms, including a rash, and is typically associated with exposure to contaminated water. It's rare but can be severe, making it a consideration in the differential diagnosis for a target rash, especially with a relevant exposure history.