Concept:
Aspirin should generally be avoided in children and teenagers with viral infections such as influenza or chickenpox.
Step 1: Aspirin risk in children.
Use of aspirin in children with viral fever is associated with a rare but serious condition called Reye's syndrome.
Step 2: What is Reye's syndrome?
Reye's syndrome mainly affects the liver and brain. It may cause acute liver dysfunction and encephalopathy.
Step 3: Why aspirin is avoided.
Because of the risk of Reye's syndrome, safer antipyretics like paracetamol are generally preferred in children.
Step 4: Checking other options.
Stevens--Johnson syndrome is a severe skin reaction.
Cushing's syndrome is related to excess corticosteroids.
Guillain--Barré syndrome is an autoimmune neuropathy.
Aspirin in children with influenza is classically associated with Reye's syndrome.
Step 5: Final answer.
Therefore, the correct answer is Reye's syndrome.
\[
\boxed{\text{Reye's syndrome}}
\]