Smoking cigarettes may increase the risk of developing tuberculosis, a study has suggested.
People were asked about their smoking habits
A team from Stellenbosch University, in South Africa, studied 2,400 people living in Cape Town.
Some 82% of those who smoked, or who had done previously for at least a year, tested positive for TB, compared with 70% in the non-smokers' group.
Writing in Thorax, the researchers say smoking might reduce the ability of the lungs to fight off infections.
All those who took part in the study were asked about their smoking habits and given a tuberculin skin test to check if they were infected with TB.
People were defined as smokers if they had ever smoked for at least a year.
Smokers were also asked the average number of cigarettes they had smoked each day.
Basic information such as age, sex, education level, body mass index, and income was also taken into account.
Of 1,309 current smokers or ex-smokers, 1,070 (82%) had a positive skin test, compared with 70% (762) of the 1092 people who had never smoked.
The more cigarettes someone had smoked over their life, the higher the risk was that they would have a positive test.
The researchers, led by Dr Saskia de Boon, from the Desmond Tutu TB Centre at Stellenbosch University, said the reason for the link was not confirmed.
But they said smoking could reduce the lungs' ability to fight off infections, including TB.
They stress further evidence is needed to investigate the association.
And they said they could not discount the possibility that the link between TB infection and smoking was merely down to socio-economic and behavioural differences.
Writing in Thorax, Dr Graham Bothamley, of the East London TB Network, based at the Homerton Hospital, said: "The association of smoking with pulmonary TB might be explained by a reduced specific immunity and possibly enhanced non-specific inflammatory response."
Dr Peter Ormerod, of the British Thoracic Society, said: "This study supports other evidence showing an association between smoking and TB risk.
"Smoking damages macrophages in the lungs, which are the first line of defence against infections.
"People who smoked would therefore be more at risk of acquiring TB."