Five players from Spain's top two leagues have tested positive for coronavirus, La Liga confirmed on Sunday.
Players in Spain's top-flight and Segunda division were allowed to carry out individual work at training grounds last week.
The move was the beginning of a staggered training programme, with the aim to restart competitive games in the middle of June.
La Liga insist identifying infected players through positive tests are part of that plan.
The five players, who have not been officially named, will now be quarantined at home and tested again "in the next few days".
They will be allowed to rejoin their club's training programme only after obtaining two consecutive negative test results.
A La Liga statement read: "Among La Liga Santander and La Liga SmartBank (Segunda) clubs, five positive cases were detected in players, all of them asymptomatic and in the final phase of the disease."
"One of the objectives of these medical tests, according to the La Liga's protocol for returning to training, according to recommendations of the CSD (Supreme Sports Council) and the Ministry of Health, was precisely to detect the denominated or asymptomatic, that is, those people who are infected and that, without presenting symptoms, they can infect other people," the statement added.
"In this way, we guarantee everyone's safety when returning to work according to the occupational risk regulations."
La Liga's training plan includes players initially carrying out work individually before then expanding into small groups and finally taking part in larger team sessions.
Barcelona's players were back for their first session on Friday while Real Madrid's will return on Monday.
The medical protocol for training is strict and includes players arriving already changed and at specific times to avoid contact with others. They wear masks and gloves, and have their temperatures taken before entering the premises. At the end of training, they take away a bag of fresh kit for the next day and shower at home.
The league have not officially announced the timing for the resumption of competitive matches although Leganes coach Javier Aguirre said last week the proposed date is 20 June.
That could correspond with the Spanish government's de-escalation programme, which includes outside events being allowed to go ahead in their fourth and final phase, as long as they are attended by fewer than 400 people.
Football matches are expected to be staged behind closed doors for several months.
La Liga president Javier Tebas has said "it is not an option" to cancel the top flight given he estimates the economic fallout would cost clubs around a €1 billion. But the third and fourth tiers in Spain will not complete their seasons, as confirmed by the Spanish football federation (RFEF) on Wednesday. Instead, promotions will be decided through play-offs and there will be no relegations.