You will not usually have any symptoms when you first become infected with the herpes simplex virus (the primary infection).

An outbreak of cold sores may occur some time later and keep coming back (recurrent infection).

However, if the primary infection does cause symptoms, they can be quite severe.

Herpes simplex virus primary infection

In children

Symptoms of the primary infection are most likely to develop in children who are younger than five years old. Symptoms include:

• swollen and irritated gums with small, painful sores in and around the mouth – this is known as herpes simplex gingivostomatitis
• sore throat and swollen glands
• producing more saliva than normal
• high temperature (fever) of 38ºC (100.4ºF) or above
• dehydration (a lack of water in the body)
• nausea (feeling sick)
• headaches 

Herpes simplex gingivostomatitis usually affects young children, but adults can also develop it. It can last between 7 and 14 days, with the sores taking up to three weeks to heal. However, gingivostomatitis does not usually recur after the primary infection.

In adults

Primary herpes simplex viruses are rare in adults, but the symptoms are similar to those experienced by children.

You will usually have a sore throat with or without swollen glands. You may also have bad breath (halitosis) and painful sores in and around your mouth. These can develop into ulcers with grey or yellow centres.

If you develop the herpes simplex virus at an early age, it may be triggered periodically in later life and can cause recurring bouts of cold sores. After the primary infection, the symptoms are usually reduced to just the cold sores themselves.

Recurrent infections (cold sores)

Recurrent infections usually last for less time and are less severe than the primary infection. The only symptom is an outbreak of cold sores, although you may also have swollen glands.

An outbreak of cold sores will usually start with a tingling, itching or burning sensation around your mouth. Small fluid-filled sores will then develop, usually on the edges of your lower lip.

If you have frequent recurrent infections, you may develop cold sores in the same place every time. They may grow in size and cause irritation and pain. Initially, they may ooze before crusting or scabbing over within 48 hours of the initial tingling sensation.

Most cold sores disappear within 7 to 10 days without treatment and usually heal without scarring.