A 28-year-old Polish woman who has been residing in the United Kingdom for four years presented to the accident and emergency department of a district general hospital with severe ulceration of the perineal area, dysuria, soreness of the mouth, fever, non-productive cough, and a facial rash. Her illness started two weeks prior to presentation with a flu-like illness and sore throat for which she was prescribed with antibiotics. Three days into the illness, she developed severe burning and itching of her genital region. Antifungal topical treatment and appropriate antibiotic therapy for a concurrent urinary tract infection provided minimal relief. She further developed high fever, vomiting, and a facial rash. Her medical history included von Willibrand's disease and a caesarean section two years prior to presentation. She had no relevant drug history or sexual history. On examination our patient was unwell with a temperature of 38°C, heart rate of 110 beats per minute, blood pressure of 105/60, and initial oxygen saturations of 100% on air. She had crepitations at the base of her left lung, tenderness on palpation at the right upper quadrant of her abdomen, and an ill-defined rash over her face and neck. There was some erythema at the back of her mouth and a white coating resembling candida. She had mild injection of both eyes. The most marked findings were of vulval and perineal erythema and excoriations with discrete ulceration. The associated pain was so severe it prevented her from moving her legs freely. Further investigations revealed that she had a lymphopenia of 0.26 × 109/L (NR = 1.10 to 4.80) and a C-reactive protein (CRP) level of 49 (NR = <5). Meanwhile, abnormal liver function tests revealed an alanine transaminase (ALT) level of 342I U/L (NR = 0 to 55) and alkaline phosphatase (ALP) level of 185 IU/L (NR = 40 to 150). With these initial findings of genital ulceration, oral soreness and candidiasis, fever, and rash, we sought an infectious disease opinion. The differential diagnosis at this stage included human immunodeficiency virus (HIV), herpes simplex virus, and noninfective causes such as Bechet's, systemic lupus erythematosus, and systemic vasculitis. Reiter's syndrome was also considered as it can present without arthritis in women. Tests for viral screen (including measles IgM and IgG), anti-streptolysin O (ASO) titre, HIV, PCR, complement levels, autoimmune screen, and immunoglobulin levels were requested. Our patient, meanwhile, was initially treated with intravenous acyclovir and high-dose prednisolone. Within 20 hours of presentation she developed type 1 respiratory failure with a pO2 of 9.5 KPa on 10 liters of oxygen. She began to tire and required noninvasive ventilation. Her respiratory symptoms were out of proportion to the changes exhibited in her chest X-ray (minimal consolidation at her left lung base, see Figure ), although clinically she began to develop bronchial breathing at her left lung base. The most likely diagnosis at this stage was HIV seroconversion complicated by Pnemocystis carinii pneumonia. Another possibility was of an atypical but severe bacterial infection. She was treated with intravenous broad spectrum antibiotics (imipenem and teicoplanin) and treatment doses of cotrimoxazole. Her steroids were also changed to high-dose methylprednisolone. On day 4 of admission her facial rash had resolved and her oxygenation began to improve (PO2 13.1 Kpa, Fio2 at 40%). Meanwhile, her blood tests revealed negative HIV PCR, negative ASO titre, normal complement levels, normal immunoglobulin levels, and negative autoimmune screen. Her steroid medication was slowly weaned and the antibiotics, including cotrimoxazole, were stopped. Her symptoms had completely resolved by day 10 of admission. It seemed likely that she had an atypical viral infection. Elevated serum titres of IgM and IgG antibodies to measles were subsequently demonstrated, thus confirming a diagnosis of acute measles. The patient is now well. A vaccination history of our patient revealed that she had completed all her childhood vaccines in Poland.