This is from the AMA
Acute illness with systemic symptoms: An illness that causes systemic symptoms and has a high risk of morbidity without treatment. For systemic general symptoms, such as fever, body aches, or fatigue in a minor illness that may be treated to alleviate symptoms, shorten the course of illness, or to prevent complications, see the definitions for self-limited or minor problem or acute, uncomplicated illness or injury. Systemic symptoms may not be general but may be single system. Examples may include pyelonephritis, pneumonitis, or colitis.