Final answer:
The most likely infectious organism causing sudden facial diplegia in a boy at summer camp is Borrelia burgdorferi, the bacteria responsible for Lyme disease.
Step-by-step explanation:
An 8-year-old boy presenting with sudden onset of facial diplegia after spending two weeks at a summer camp is most likely to have been infected by Borrelia burgdorferi, the causative agent of Lyme disease. Lyme disease can cause a range of symptoms including facial palsy, flu-like symptoms, joint pain, and even neurologic complications if left untreated. Spending time outdoors, especially in wooded areas, increases the likelihood of tick encounters, which are known vectors of Borrelia burgdorferi. Early diagnosis and treatment are imperative with Lyme disease to prevent serious health complications.