Concept:
Pharmacists must identify warning signs that require referral to a doctor.
Step 1: Mild headache, common cold, and nasal congestion may often be managed initially with OTC advice.
Step 2: Fever lasting more than 3 days may indicate serious infection or underlying disease.
Step 3: Such a case should not be treated casually.
Step 4: Hence, immediate medical referral is required.
\[
\boxed{\text{Fever persisting beyond 3 days}}
\]