Thirteen Causes Of (And Contributors To) Oral Thrush

Oral thrush is a relatively common disease affecting nearly 5% of all infants1, and in all ages is the most common fungal infection of the mouth2.

In adults it is usually associated with some predisposing factor, like conditions which weaken the immune system, for example.  Also known, among many other names, as candidiasis, it is often caused by Candida albicans.

A fungal infection of the soft tissues of the mouth, it usually affects the tongue or soft mucosa of the cheeks, which take on a white furry appearance, sometimes described as cottage-cheese-like.

Oral thrush at back of mouth

Image of oral thrush, adapted from original photograph from this source

Candida fungi are extremely common in the normal microbial flora of the mouth and are present in around 50% of people3.  Not everyone gets oral candidiasis though, and there is a circle of processes which normally keep the fungus in check, as indeed they keep all of the microbial flora at bay.

A quick answer to a common question here – “Is oral thrush contagious” – No it is not contagious. Although it is an infection, it is not spread from person to person. As noted above, many people carry the fungus as part of their “microbial load” – It is only when other factors come into play does the fungus spread to cause infection.

The factors that normally protect us are saliva, which form an important part of the immune system in the mouth, competition from other less dangerous bacteria also present, and of course the lining of the mouth, called the epithelium.  It is damage to, or an imbalance in one of these three things which usually results in someone developing the infection.

By looking at them in turn we can understand better some of the 13 most common causes of oral thrush.


mouth candidiasis baby

A baby’s tongue affected by candidiasis. Image credit

The immune system

1.  Babies are not born immune to Candida, and their immune systems take a long time to mature, so about 5% of babies will get the condition. It usually lasts for up to 2 weeks and does not usually require treatment.

2.  The elderly also have a weak immune system, however, and commonly suffer from candidiasis.  There are also other reasons why they can be more likely to get it, as we shall see, and so are a particularly susceptible group. 

3.  Drugs which affect the immune system naturally lead to an increase in incidence of fungal overgrowth in patients that are taking them.  Patients taking steroids, such as transplant patients for example, are therefore at risk. Another cause is from steroid inhalers needed by some asthmatics.

Steroid inhlers

Steroid inhaler use may lead to candidiasis

4.  And for some patient’s, the immune system itself can be damaged or inactive for a variety of reasons, of which the most obvious is HIV/AIDS, and a large number of these patients show signs of oral candidiasis.



5. There are many drugs which can affect the normal production of saliva, and chemotherapy is one of those causes.  Radiotherapy to the head and neck area frequently disrupts saliva production.  To add to this, chemotherapy can also directly affect the immune system.  For these reasons cancer patents are at a higher risk of thrush compared to others.

6. Dry mouth due to other causes therefore has a knock-on effect which can result in candidiasis of the mouth.  The causes of dry mouth are wide however, and even whether a patient has it or not is extremely subjective, with many patients with normal saliva production reporting symptoms of dry mouth.  The link between thrush and xerostomia, therefore, is not as strong as with other causes.


Mucosa (epithelium)

7. Patients who wear dentures are more likely to get thrush, mainly due to the fact that the materials used in their construction are reasonably suitable for Candida fungi, and the moist environment that it offers. Denture hygiene and removal at night is very important. If you use any adhesive on your dentures, make sure this is cleaned off every night.

"Clean dentures in water"

Keeping dentures clean and removed at night is a must!!

8. In another blow for the elderly, dentures which are poorly made and cause trauma to the mucosa can also let oral thrush start in this localized area of damaged tissue.

9. There are also diseases, which can occur in all ages, which change the shape of the protective epithelium, and this can obviously lead to thrush developing more easily.  The types of change which lead to growth of the fungus are dysplasia and hyperkeratosis, among others.  Diseases which cause these changes include leukoplakia, which can commonly be mistaken for oral thrush, and lichen planus among others.


The balance of microbes

10. A very common reason for people to get oral thrush is treatment with antibiotics, which is due to the balance between less harmful bacteria and Candida fungus being upset.  Although common, it is usually short lived and easy to treat, especially after treatment with antibiotics ceases.


Other factors

11. Smoking is also associated with this disease, although the mechanism by which this happens is poorly understood.

12. Malnutrition, although not a common cause in Western countries, is also associated with oral candidiasis. This can happen either as a result of a weakened immune system, or as a result of the changes which happen to the mucosa during deficiency.

13. There are also a range of other diseases which are either commonly mistaken for candidosis, or are so similar that some experts regard them as the same disease.  This group includes denture related stomatitis, angular cheilitis, median rhomboid glossitis among others.  Although not direct causes of oral thrush, people with these conditions are strongly linked with cases of it, if not mistaken for having it, because many of the predisposing factors are also present in this group too.



In all, oral thrush is a fairly commonly seen condition, which, while not serious, can be painful and unpleasant for the sufferer.  In many cases it is the fact that many other things are associated with it, that makes this infection difficult for this group of patients.

By asking what causes oral thrush for the individual, we can set about eliminating these potential causes one by one -which can often be key to recovery, without any further treatment being needed.

Next: Read more on the treatment of the problem here.



  1. Soames JV, Southam JC, (1999). Oral Pathology (3rd Edition). Oxford: OxfordUniversity Press. pp. 147, 193-200.  ISBN 0192628941
  2. Bouquot, Brad W. Neville, Douglas D. Damm, Carl M. Allen, Jerry E. (2002). Oral & Maxillofacial Pathology (2nd Edition).  Pholadelphia: WB Saunders. pp. 189-197.  ISBN 0721690033
  3. Kerawala C, Newlands C (editors) (2010).  Oral and maxillofacial surgery.  Oxford: OxfordUniversity Press. pp. 446, 447.   ISBN 9780199204830

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