A new study says babies exposed to passive smoking are almost twice as likely to need hospital treatment for serious infectious diseases.
Researchers in Hong Kong say exposure to second-hand smoke may weaken children's immune systems.
The study followed almost 7,500 children born over a two-month period in the late 1990s until they were eight years of age.
Researchers from the University of Hong Kong say those who had been exposed to smoke in the first six months of life were 45% more likely to be admitted to hospital for treatment of an infectious disease by the age of eight.
One in three children who had someone smoking within 3m of them during their first few months ended up in hospital. Children with low birth weights were particularly at risk.
As well as causing respiratory problems, the study says the increased risk of infection for all infants exposed to passive smoke suggests it could weaken the development of their immune systems.
