Viral Gastroenteritis with Secondary Rash: This is a common condition where a viral infection causing gastrointestinal symptoms (stomach bug) is followed by a rash, which can be due to the viral infection itself or a secondary bacterial infection. The rash is often not specific and can be seen in various viral infections.

Measles: Although less common in areas with high vaccination rates, measles can start with prodromal symptoms that include fever, cough, runny nose, and conjunctivitis, followed by a characteristic rash. The initial symptoms could be mistaken for a stomach bug.
Roseola: Caused by human herpesvirus 6 (HHV-6) or human herpesvirus 7 (HHV-7), roseola typically starts with a high fever, which may be accompanied by mild gastrointestinal symptoms, followed by a rash once the fever resolves.
Enteroviral Infections: Enteroviruses can cause a range of symptoms including gastrointestinal distress, fever, and rash. The presentation can vary widely depending on the specific virus and the individual's immune response.

Meningococcemia: This is a severe infection caused by Neisseria meningitidis, which can present with fever, rash, and gastrointestinal symptoms. The rash associated with meningococcemia is typically petechial or purpuric and can be a sign of sepsis, making prompt diagnosis and treatment critical.
Stevens-Johnson Syndrome/Toxic Epidermal Necrolysis (SJS/TEN): Although rare, these are severe skin and mucous membrane disorders, usually a reaction to medication or infection. Early recognition is crucial as they can be life-threatening.
Kawasaki Disease: An acute febrile illness of childhood characterized by inflammation in the walls of medium-sized arteries throughout the body, which can present with fever, rash, and gastrointestinal symptoms. It is a do-not-miss diagnosis due to the risk of coronary artery aneurysms if not treated promptly.

Erythema Infectiosum (Fifth Disease): Caused by parvovirus B19, it typically presents with a "slapped cheek" rash and can be accompanied by mild prodromal symptoms, including fever and gastrointestinal upset.
Infectious Mononucleosis (Mono): Caused by Epstein-Barr virus (EBV), it can present with fever, sore throat, and rash, especially if the patient is taking ampicillin or other penicillins. Gastrointestinal symptoms can occur but are not the primary presentation.