Tests showed that neither the water in the pools, nor the drinking water nor the food served in restaurants at the Mon Repos aquatic centre were the cause of ailments suffered by the swimmers on Sunday, the city said on Wednesday.
The victims were competitors in Swiss national swimming championships, about half of whom were treated in hospital after suffering from vomiting and stomach cramps.
The city closed the swimming complex on Monday until the results of medical tests could be known.
An investigation led by hospital services and the cantonal doctor identified the cause of the bouts of illness as a highly contagious norovirus susceptible to being spread rapidly in the heat and humidity of the complex, the city said in a news release.
The condition of the victims was followed up and everyone is "out of danger", Patrice Iseli, chief of the Lausanne sports service, told the ATS news agency.
"In the majority of cases it was not particularly serious but bothersome."
The virus originated from outside the complex and spread through human contact, authorities believe.
The pool, set to reopen on Thursday, has been completely disinfected and presents no risk to the public, the city said.