Written by Staff Writer

CNN U.S. Health Editor Elizabeth Cohen.

The coronavirus is most effective against old or people with immuno defficiencies, particularly those over 60, researchers say. In the wild, the virus has caused 54 cases. Nine have been fatal, the World Health Organization says.

The findings by researchers in The Netherlands suggest that post-exposure antigens of the coronavirus caused sick people to shed significantly less virus once they recovered from it.

"Thus far, the presence of the human coronavirus is extremely important in a disease outbreak," the researchers write in the latest issue of the Journal of Clinical Virology.

Previous research has shown the virus attacks weakened immune systems, but this is the first research to quantify that effect.

The latest study, a retrospective review of 49 cases found only one virus-free patient - all of them people infected before they were hospitalized.

The most affected age group was 60-69. The likely route of infection was travel, the authors note. The man from Saudi Arabia who was the first death this year had flown in and out of London between Sept. 30 and Oct. 1, the WHO said last week.

The virus belongs to the same family as SARS, which triggered a massive global epidemic in 2003. It is different in that SARS was a coronavirus that is transmitted person-to-person, not as a spore, which means it cannot pass from an animal to a human.

SARS can't infect people from other animals. The virus does cause inflammation of the blood vessels in the skin, but not in the heart.

Dr. Peter Piot, who heads the London-based Wellcome Trust Global Health Institute, told CNN's "New Day" this week that it could be decades before we know what path the coronavirus has taken.

"It's a virus that has never been found in camels," Piot said. "This is so new ... they're only now starting to look for it in camels.

"If we look for the pathogen in camels, we will find it."