NHS Direct is handling more than 4,000 calls a week from people suffering from suspected norovirus, figures showed today.
The number seeking help for diarrhoea and vomiting has been rising in recent weeks as the virus begins to take hold throughout the UK.
In November, NHS Direct received a total of 17,700 calls about these symptoms, which are most often caused by norovirus.
It is the highest number of calls per month since March and is in line with expectations for the time of year.
The Health Protection Agency (HPA) has analysed the data for NHS Direct's call volume and found it is consistently high enough to suggest the "beginning and continuation of periods of norovirus activity nationally".
Separate data from the HPA shows there have been 91 suspected outbreaks of norovirus in hospitals since the summer, 74 of which have led to ward closures.
The HPA said the number of reports from hospitals is rising but is in line with expectations.
Most people with the bug will make a full recovery within two to three days and are advised to stay at home, take paracetamol to relieve fever and drink plenty of water to replace lost fluids.
Drinking fluids is particularly important for vulnerable groups such as young children and the elderly.
NHS Direct has launched an online health and symptom checker for people suffering from diarrhoea and vomiting.
Based on the questions asked by NHS Direct call handlers, it guides people through their symptoms and offers advice on how they can treat them with over-the-counter medicines.
Patients may be told to seek further help and there is a "call-back" option if the person needs to speak to a nurse.
Helen Young, chief nurse at NHS Direct, said: "It's not unusual to see an increase in the number of people contracting norovirus at this time of the year and NHS Direct has vast experience of helping people manage the unpleasant symptoms.
"In most cases people can follow advice that allows them to treat their symptoms at home and this new health and symptom checker is a great way for patients to get the advice they need to help them get better as quickly as possible."
The checker is at http://www.nhs.uk/nhsdirect.Reuse content