"Seven players have somehow come down with slight cases of the flu," Loew was quoted telling Germany's ARD radio. He said that most had "throat aches" but did not reveal their names.
"As a result, it's too early to make any final decisions about the lineup," Loew added.
Defender Mat Hummels missed Germany's round of 16 match against Algeria on Monday due to a fever. He said on Wednesday he had recovered and posted a picture of himself sitting on the beach.
Reserve midfielder Christoph Kramer was suffering from a case of the chills on Wednesday and was the only player who did not take part in the training session, said assistant coach Andreas Koepke, who added all other players were fit and ready to travel to Rio de Janeiro for Friday's match
Loew has said that the travel across Brazil and different climate zones for their four matches so far as well as matches in tropical heat, a heavy downpour and then in chilly Porto Alegre - along with air conditioning - had taken their toll.
"I hope they're all doing well now," Loew said. He added: "It's not all that bad at the moment. I don't want to dramatise it."
But Loew said that Hummels condition had actually deteriorated. "Mats Hummels suffered a negative setback on the second day," Loew said.
Loew has come under heavy fire in Germany for his decision to put captain Philipp Lahm in midfield instead of at right back, where he has excelled for years. Loew was forced to put Lahm at right back late in the Algeria match due to injuries.