The streets around the Rockefeller Centre where NBC has its offices throng with people. Some stop to watch a silent newscast through a shop window.

"We found out about it on the news too," says Randy Savitch, a stagehand on NBC's Nightly News.

"The managers didn't tell us because they didn't want people fleeing the building."

The spectre of bio-terrorism has haunted Americans since the 11 September attacks, with sales of gas masks rocketing.

A female NBC employee appears to have contracted a different - and less serious - strain of anthrax to that seen in Florida and the woman, who may have come into contact with the disease two weeks ago, is responding to treatment.

"She's going to be fine. Now she's more concerned about her co-workers," says Mr Savitch.

White powder

Officials suspect the woman may have become infected after opening a letter addressed to NBC Nightly News anchorman Tom Brokaw.

The envelope contained a white powder - similar to that thought to have caused the Florida outbreak.

Although this substance later tested negative for anthrax, Mr Brokaw's assistant went on to develop a skin rash.

The NBC mailroom and an office where the letter was taken have been evacuated and thoroughly tested.

Mail problem

Joanne Rullan, who sorts letters at the NBC building says she has been badly shaken by events.

"I don't know what's going to happen on Monday when the mail arrives. I don't want to touch it."

The return of the woman's positive test results came just hours after the NBC building experienced a bomb scare.

Nic Imelio, who works on NBC's popular Conan O'Brien talkshow, says it is another blow to the city's already flagging morale.

"A month ago we stood out here and watched the World Trade Centre burning. Now I have to tell my mom not only is there anthrax in New York, it's actually in my building."

Tim Stafford, whose employer shares NBC's offices, is equally concerned by the outbreak. "It's getting a little close, but I'm not leaving work and going home because of it."

Antibiotics

Workers have been offered tests for the disease, while being reassured that anthrax cannot be passed from person to person.

In the wake of the Florida outbreak, many people across the US have tried to secure supplies of the anti-anthrax drug Cipro, despite concerns from some physicians that taken without supervision and in the absence of symptoms the antibiotic's side effects could do people more harm than good.

Mr Stafford has already contacted his doctor. "I arranged to get a prescription just to be on the safe side," he said.

"However, my pharmacist says he's having to ration Cipro because there's been such a big run on it lately."