Researchers suspect overcrowded housing is behind a near doubling of hospital admissions for infectious diseases that are linked to close human contact.
The number of admissions for these diseases, which include many respiratory, skin and gut infections, rose by 89 per cent between 1990 and 2001. By then they numbered more than 40,000, and have remained fairly static since.
People sick with these conditions increased from 8.5 per cent of hospital admissions at the start of the period, to nearly 14 per cent at the end.
"It's a big jump," said public health specialist Associate Professor Michael Baker, of Otago University's Wellington School of Medicine and Health Sciences.
"You would expect that in a modern, organised society infections should be declining, particularly these ones that are transmitted from person to person.
"The hypothesis is that we've had an increase in household crowding and increasing socio-economic inequality, with some populations particularly, in the 1990s."
Deaths in hospital from close-contact infectious diseases also rose sharply, from 550 in 1990 to 884 in 2001.
But hospital admissions for other infections rose only slightly during the 1990s.
"Although there was a large increase in hospitalisations in the 1990s, it was pretty much driven by the increase in close-contact infectious diseases. It represents a tidal change in the infectious disease burden in New Zealand," said Professor Baker, who will give a lecture on the topic at the university's Auckland centre on Thursday.
He said census data showed household crowding had declined over several decades for the total population, but had increased for sub-groups, in particular those living in the most-deprived areas.
The percentage living in multi-family households had risen in the 1990s, particularly in private rental housing in Auckland.
An underlying factor of the increase in hospital admissions for close-contact infections was thought to be the shift to market rents for state housing in the 1990s, before income-related rents were re-introduced following the change of Government in 1999.
"It's one of the theories put forward to explain this trend in the emergence of the meningococcal epidemic. That research hasn't been done yet to see if these events did coincide more precisely."
In a separate, just-published study, Professor Baker and colleagues have shown that overcrowding increases the risk of tuberculosis.
Since 2001, Housing New Zealand has carried out more than 5000 housing improvements under its Healthy Housing project. These include extending homes for larger families, and installing insulation, ventilation and heating systems.
The project has been found to increase the general health and wellbeing of the affected tenants.
Otago University has also shown benefits for health and wellbeing from installing home insulation in low-income areas.
Its research, published in the British Medical Journal, found that those in homes that had insulation installed had less wheezing, fewer days off school and work, and fewer GP visits than those in uninsulated homes.
* 21,650 people were admitted to New Zealand hospitals in 1990 with infectious diseases linked to close contact with other people.
* By 2001, the figure had risen to 40,830.