A statement on the official Wales Twitter account said: “Ben Davies and David Brooks have had to withdraw from the squad due to illness. Brysiwch wella hogia! (get well soon)”.
The loss of Tottenham defender Davies and Bournemouth forward Brooks has added to Wales’ problems before Friday’s crunch trip to the Czech Republic.
Manager Robert Page was already without injured captain Gareth Bale who had been due to win his 100th cap in Prague.
The Real Madrid forward has suffered what Page has described as a “significant hamstring tear” and is doubtful for next month’s closing World Cup qualifiers at home to Belarus and Belgium.
Defenders Rhys Norrington-Davies and Tom Lockyer dropped out of the squad on Monday through injury and were replaced by Ben Cabango and Will Vaulks.
Wales enter their away double-header level on points with the second-placed Czech Republic and with a game in hand.
Belgium are almost certain to win the group, leaving the Czech Republic and Wales in a battle for second spot and the World Cup play-off place which comes with it.
After their trip to Prague, Wales play Estonia in Tallinn on Monday.