The trends parallel the proportions of children in "poverty" - defined as living in homes with less than 60 per cent of the median income after housing costs.
Child poverty also doubled in the 1990s, from 16 per cent of children in 1990 to 30 per cent in 2001. It then fell to 22 per cent by 2007 as employment grew and Working for Families tax credits gave a fillip to low-income families, but has risen again to 26 per cent in the current recession.
Across the whole 20-year period for adults as well as children, Dr Baker's team has calculated that an extra 22,000 people are now admitted to hospital every year who would not have needed hospital treatment if the rates of close-contact infectious diseases had stayed at 1991 levels.
His team has done a case study of rheumatic fever and found that rates were 23 times higher in the fifth of census area units with the highest rates of household overcrowding than it was in the least crowded fifth. Overcrowding was a stronger driver of the disease than any other factor.
"It has become almost exclusively a Maori and Pacific disease," said Dr Baker.
He says infectious disease rates are also affected by the cost of going to a doctor, and by weakened immunity caused by poor diet and other stresses of poverty.
Papakura couple Carla and Craig Bradley pay $36 to see their doctor for any adult issue and often delay both going to the doctor and picking up prescriptions until they can afford it.
"I had to put off my medication for post-natal depression and contraceptive pills," says Carla. "It's like $3 or something, but that could go towards milk."
In contrast, St Heliers graphic designer Nigel Smith, who has inherited type 1 diabetes, has recently gone on to a small insulin pump worn on his belt which maintains his blood sugar level continuously, removing the need to inject himself with insulin twice a day. He said the Auckland District Health Board paid for the pump but not the consumables, which cost about $2500 a year. "I wonder how someone who is unemployed would cope."
Some health programmes have made a dent in health disparities. A "Healthy Housing" programme in 6000 Housing NZ homes in Manukau and elsewhere cut acute admissions by 11 per cent for preschoolers and by 23 per cent for older children and young adults. Health and housing officials assessed each home and fixed any problems by actions such as installing insulation or moving families to bigger homes.
Housing NZ funding for the programme ended last July, but Counties-Manukau District Health Board still funds referrals to healthcare.
A programme targeting rheumatic fever through school-based sore throat clinics will get $24 million over the next four years under the Maori Party's coalition deal with the National Party.
Dr Baker says: "Maybe we should be using the same approach to deal with all infectious diseases in children."
But he says "the big fundamental issues" are inequality and housing.
"A basic thing would be a housing warrant of fitness that covers health, safety and sustainability issues, a bit like the five-star approach with appliances." He believes that could be run by the Auckland Council.
