The tonsils are small glandular structures located on either side of the back of the throat that provide a first line of defence against microbes. Most viruses and bacteria access the body via the nose and mouth. Healthy tonsils filter out these bacteria and also produce defensive immune cells against the invading microbes.

These defensive blood cells (macrophages) present in the tonsils engulf the infective organisms and secrete digestive enzymes that destroy them. If the infection is serious, this can lead to pain and inflammatory changes such as redness, soreness and fever. Fever is actually a sign that the body's immune defences have been activated. This immune function of the tonsils starts to decline after puberty, which may be why tonsillitis is so rare among adults. Tonsillitis is most common between preschool age and the mid-teenage years. Children of school age are exposed to bacterial or viral pathogens on a frequent basis, which also increases the likelihood of tonsillitis in this age group.

Tonsillitis is usually caused by viral infection, although bacterial infection can also be the cause. The tonsils proper may be affected or the throat and surrounding areas including the back of the throat or the pharynx may be involved. It is rare for bacterial infections to cause tonsillitis. One of the most commonly feared bacterial causes of tonsillitis is infection with Group A streptococcal bacteria, which can result in strep throat. Other types of bacteria that can cause tonsillitis include Hemophilus inflenzae and neisseria.

Of the viruses, two of the most common causes of tonsillitis are adenovirus and influenza. Other causative viruses include rhinovirus (also causes the common cold), parainfluenza virus, enteroviruses (cause hand, foot and mouth disease), adenoviruses (cause diarrhoea), the measles virus and Epstein-Barr virus (causes glandular fever).

Tonsillitis may also be caused by an over-reactive and aberrant response of the immune system to the normal bacterial environment in the mouth and the throat. This is the reason why some people are more prone to tonsillitis than others.

Bacterial tonsillitis

A person who suddenly becomes unwell with a fever and develops a severe pain in the throat, may well have bacterial tonsillitis, which is also referred to as strep throat. Streptococcus pyogenes is a type of bacteria that exists harmlessly in the nose and throat in around 15% of healthy individuals. These people have no symptoms of strep throat but are classed as carriers of the infection and can pass the bacteria onto others.

Although, this bacteria usually exists in the throat and mouth without causing any harm, the “bug” can start to cause symptoms if the immune system is under strain. If a person is stressed, exhausted or already off a virus, for example, the immune system may be weakened, which allows toxins and inflammatory substances produced by the strep organisms to suddenly start causing symptoms such as a sore throat, bad breath, difficulty swallowing, red and swollen tonsils and white pus on the tonsils.

Bacterial tonsillitis usually clears up within a few days without requiring any treatment. Painkillers can be used to relieve symptoms while the immune system fights off the infection.

Infectivity

Depending on what is causing the tonsillitis, this condition may or may not be contagious. If it is caused by viral infection, tonsillitis is usually contagious, although it may not be if a person happens to have been exposed to that virus previously.

Tonsillitis is also highly contagious when it is caused by bacterial infection. In cases where the condition is caused by chronic illness such as allergic rhinitis or sinusitis, for example, tonsillitis is unlikely to infect others.