What's the difference between a cold and flu?

Colds start gradually with runny nose and sore throat and mild systemic symptoms. Flu starts abruptly with high fever, marked body aches, headache, and exhaustion. Flu is far more likely to cause complications requiring hospitalisation.

A practical distinction: flu tends to arrive abruptly with fever and body aches severe enough that you stop what you are doing. Colds build over a day or two and let you continue.

Antivirals such as oseltamivir are most effective within 48 hours of symptom onset, which is why early assessment matters for high-risk people.