A second case of measles has been reported in quick succession with South Australian health authorities warning at-risk individuals to be on alert amid a potential wider outbreak.

A 17-year-old boy from Adelaide is in a stable condition in hospital after being exposed to an infected infant who became ill after returning from overseas.

"Measles is a serious and highly contagious infectious disease," SA Health said.

Measles is spread in the air through coughing or sneezing by someone who is unwell with the disease.

Symptoms include fever, sore eyes and a cough followed three or four days later by a red, blotchy rash which typically spreads from the head and neck to the rest of the body.

It can take up to 18 days for symptoms to appear after exposure.

Anyone born in or after 1966 who has not received two doses of measles vaccine is advised to get vaccinated.

SA Health advised anyone who isn't fully immunised, and particularly who may have come into contact with a positive case, to be alert for symptoms and see a doctor if they become ill.

Locations visited by the latest infected case include a medical centre in Norwood on April 2 and the Women's and Children's Hospital emergency department on April 3.

A full list of public exposure sites, stretching back to March 15, are available on the SA Health website.

It is the third reported case of measles in South Australia so far this year after a one-year-old child tested positive in February.