Other staff run a bookmobile and do the rounds of schools and day-care centres.

ECONOMIST: Books in Wyoming

Some 13% of children under three go to day-care centres, compared with 54% in America.

ECONOMIST: Japan

Two income families are becoming the norm, and parental responsibilities are increasingly shuffled over to schools or day-care centres.

ECONOMIST: The 20th century

According to the Tokyo government's own statistics there are 20, 000 children in the city waiting for places in day-care centres.

BBC: Japan: The worst developed country for working mothers?

At its simplest, parenthood exposes even the affluent middle classes to public services, from hospitals to day-care centres or libraries.

ECONOMIST: Bagehot

This covers everything from the people who work in the cafeterias and day-care centres on military bases to accountants and the managers of health-care plans for employees.

ECONOMIST: Defence spending

Ms Bachelet has started to build new day-care centres and has promised universal nursery schooling by the end of her four-year term, which will help mothers who want to work.

ECONOMIST: Will Michelle Bachelet help women or hinder them?

The researchers found that similar strains of bacteria were found within individual day care centres - but that different centres harboured different strains.

BBC: Nursery