Norovirus, better known as the winter vomiting bug, is highly contagious and the most common kind of stomach bug in Ireland.
The HSE warns that between 10,000 and 20,000 can catch it per week.
The main symptoms include nausea, diarrhoea and vomiting, and you may also have a high temperature, aching limbs and a headache.
They start within one to two days of being infected.
To avoid catching norovirus, you should avoid close contact with anyone who has it, disinfect any surfaces touched by them, wash your hands with soap and water frequently (alcohol hand gels won't kill the virus) and avoid sharing towels.
But what should you do if it's too late and you've already caught the bug? Dr Hazel Wallace shared some advice.
"Although the symptoms are unpleasant they are not often harmful and the virus will clear up itself in a couple of days," she said.
"Dehydration is the main risk associated with norovirus because the body is losing water frequently as a result of vomiting and diarrhoea.
She added: "You can avoid dehydration from norovirus by drinking plenty of fluids. You will need to drink more fluids than usual to replace the water that has been lost, and you should avoid drinking fizzy drinks and fruit juices as these can dehydrate you further and may make diarrhoea worse.
"It’s important to replace lost electrolytes after bouts of sickness and diarrhoea. This can be done by taking an oral rehydration solution from your pharmacy. These products contain the right balance of salts and sugars to rehydrate the body effectively."
Your pharmacist can advise you on more treatments for dehydration, such as an oral rehydration therapy.