Staffers and patients at the Hospital de San Carlos have donned face masks in the wave of deaths from an unknown cause.
Although one man who died was in his 70s, the ages of the other four ranged from 18 to 54.
Hospital officials and those from the Caja Costarricense de Seguro Social are dismissing rumors that swine flu is the culprit. Instead, some of the cases have been blamed on letrospirosis, a bacterial disease that can come from domestic and wild animals, including rodents.
Those who died lived in geographically dispersed homes, so there does not seem to be a single cause.
Physicians have been delayed in making a clear diagnosis because the public agency that usually does that type of work was on a holiday break. Some staffers returned to work Tuesday in light of the outbreak.
The agency is the Instituto Costarricense de Investigación y Enseñanza en Nutrición y Salud in Tres Ríos, Cartago.
Deaths from respiratory ailments are not unusual at hospitals.
And there are as many as 15 patients under care in San Carlos now with such problems.
The deaths have attracted the attention of top Caja officials and the Judicial Investigating Organization that keeps track of deaths without established causes. Health experts have gone to the hospital.
Leptospirosis is generally spread by animal urine entering the water supply. The U.S. Centers for Disease Control said that leptospirosis is caused by bacteria of the genus Leptospira.
In humans, it can cause a wide range of symptoms, some of which may be mistaken for other diseases, but some infected persons, however, may have no symptoms at all, it added.
Without treatment, leptospirosis can lead to kidney damage, meningitis (inflammation of the membrane around the brain and spinal cord), liver failure, respiratory distress, and even death, said the Centers.
The first death reported at the Hospital de San Carlos was last Wednesday.