Health officials reported this week that as many as 50 new SARS cases had surfaced in Toronto, prompting the World Health Organization to put the city back on its list of infected areas. The outbreak came two weeks after Toronto was declared SARS-free. More than 1,400 people were quarantined as doctors traced the virus spread to patients in four hospitals. At the height of the crisis, severe acute respiratory syndrome infected more than 290 people in Toronto, killing 27. Toronto began battling the disease in March, when a sick woman arrived on a plane from Hong Kong. Until the disease is eliminated everywhere, said one WHO official, it is going to be a threat everywhere.