A case of measles. Credit:US Centres for Disease Control and Prevention Dr Roscoe Taylor, the state's acting chief health officer, said authorities were concerned more people may have already contracted the illness after coming into contact with the four patients. "The number of cases potentially could be very large," Dr Taylor said. "And of course the concern always with measles is that the next wave of cases, if they occur, could have an ever wider circle of contacts. "It's very possible over the next week or so those people who were actively infectious to others in the last week could have transmitted it, and people could be incubating it."

The measles rash on the face of a child. Credit:Centres for Disease Control and Prevention Most concerning is that none of the four people diagnosed are believed to be the so-called "original patient zero". It is believed they caught it from someone else in the community, who may have contracted it overseas. Dr Taylor said it was impossible to determine who they might have had contact with or track the likely path of the infection. Measles is a highly-infectious virus that lives in the nose and throat mucus of an infected person and spreads through coughing and sneezing.

The virus can also live for up to two hours in an airspace where the infected person has coughed or sneezed. People can contract and spread the virus for several days before symptoms - including fever, sore throat, red eyes and coughing - begin to show. The characteristic measles rash usually appears three to five days after the first symptoms develop. Anyone developing those symptoms is urged to call ahead before visiting a GP to enable the spread of infection to be minimised. Children and adults under 50 who have not had two measles vaccinations are most at risk. Authorities have said they should strongly consider getting vaccinated immediately.

Children who have had both courses of the vaccine are extremely unlikely to be at risk. The GPs who treated the four diagnosed measles patient are contacting anyone who may have had contact with them in their clinic waiting rooms and the health department has issued an alert to doctors across the state asking them to look out for further infections. Dr Taylor dismissed concerns about the virus striking in Brunswick, which is stereotyped as a more alternative area. "The data for children's vaccinations in measles for that area isn't bad," he said. "It's a little lower than the state average but it's certainly not something to be overly concerned about.

"We're seeing this more in younger adults. That group grew up in a time when there wasn't two doses of measles vaccine being provided." The measles vaccine, which is administered as the measles, mumps and rubella (MMR) vaccine, is part of the National Immunisation Program for children between 12 months and four years of age. Dr Roscoe Taylor. Credit:Liam Mannix Women in their 20s to 40s can receive the MMR vaccine free of charge under the Victorian government's initiative to ensure women of child-bearing age are protected against rubella. Meanwhile, people under 20 can also receive a free MMR vaccine under the federal government's catch-up campaign.

A community's immunisation rate has to be about 90 per cent for what is known as 'herd immunity' to kick in, according to the National Centre for Immunisation Research and Surveillance. Herd immunity occurs when the level of immunisation is high enough to create a ring of protection around the community against a particular disease. It is crucial to protect the community's most vulnerable, including newborn babies. However, an even higher immunisation rate is needed for measles, for which 95 per cent coverage is optimal. Late last year, a Brunswick school was at the centre of a chickenpox outbreak.

One in four students at Brunswick North West Primary School contracted chickenpox within a fortnight. Only 73.2 per cent of children at the school were fully immunised at the time, although the overall immunisation rate for the suburb was closer to the state average of 93 per cent. Under the new No Jab, No Play laws which came into effect on January 1 this year, all Victorian children must be fully vaccinated to attend childcare and kindergarten. The groups of people most at risk of catching measles are: Anyone who is unvaccinated;

Adults between 35 and 49 years, because many in this age group did not receive measles vaccine;

People at any age who are immunocompromised, even if they have had measles, or have been immunised. This includes people with diseases such as cancer and people who are undergoing cancer treatment or are on high-dose steroids.