Hard-hitting Queensland forward Nate Myles has been struck down by a stomach virus on the eve of Wednesday night's State of Origin showdown with NSW at ANZ Stadium.

Myles was isolated from his teammates and missed Queensland's final training run on Tuesday afternoon.

Maroons teammate Dallas Johnson was also understood to be feeling off colour although the crash-tackling lock took part in the team's last hit-out.

Myles, who played a huge role off the bench with 22 tackles and 12 hit ups in Queensland's 28-18 first game win in Melbourne last month, was being treated by Maroons team doctor Roy Saunders.

At this stage he is still expected to play unless his condition deteriorates.

PNG international Neville Costigan, who has been in camp with the Queenslanders all week, is on standby for Myles.

Costigan, a similar damaging defender to Myles, played two Origins in 2007 when Queensland took the second of their three straight series triumphs.

Myles' scare is the first hiccup for coach Mal Meninga who is shooting for a history-making fourth straight series.

A few hours earlier Meninga told journalists how smoothly Queensland's preparations had been going without any injuries or illnesses.

Myles' condition will be monitored in the morning.

But with Costigan already in camp and a part of the build-up in Terrigal during the week, a decision on Myles could be left until Wednesday afternoon.

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