IT IS time to sit down and discuss the importance of bacteria in Rugby World Cup selection. Also twists, pulls, niggles, tears, concussion and all the other ailments that can lay a player low for a day or two without having any effect at all on their long-term involvement in the tournament.

Let's call them all bugs. Bugs in the gut like the ones that laid Stuart McInally and Grant Gilchrist low during two of the three warm-up games; bugs in the software that could sent a player off for six days of concussion prototcol; or bugs in the hardware like the back spasms that have kept Tommy Seymour out of action, or the hamstring pull that caused a hiccup in Fraser Brown's preparations.

When Vern Cotter and his fellow coaches sat down to pick the squad for the World Cup, these bugs loomed large in their thinking, they so nearly ended up with a flanker at lock and a hooker at flanker against Italy in Turin. They can't let that happen against South Africa or Samoa.

The idea is to pick the 23 they would like to play in the final if they got there, then pick spares for prop, hooker, lock, back row, scrum half, fly half, centre and back three. Then plug whatever holes are left.

To complicate things, some of those spares may have already justified their places as specialists. So Peter Horne could be the third fly half, though he is likely to be in the squad anyway as a centre; Sean Lamont as the fourth centre even though he will be in as a wing. It is all about doubling up.

The coaches are experts at keeping their cards close to their chest – Vern Cotter, the head coach, would not even discuss the question of whether he will take two hookers or three – and clues have been few and far between, but Matt Taylor, the defence coach, was prepared to chat about some of the principles he thinks will make the difference.

"A real target of ours has been to bring power into the game – repeatability and work rate," he said. "We worked really hard on that. A lot of the players who will find themselves in the 31 will be guys who are powerful, who bring really strong hits and power running in their game – explosiveness and speed. Speed is something we have tried to improve and a lot of the backs have got their personal bests in the last two or three weeks."

All of which plays to the strengths of the likes of David Denton and Ryan Wilson in the pack, Matt Scott and Tommy Seymour in the backs. Jim Hamilton has power but less speed, so he may be one to miss out with the more versatile Rob Harley the spare cover for lock.

The other issue for Cotter is how much he will mould the squad to counter Scotland's pool opponents, how much he will simply pick on the strengths he has and work out tactics to use what he has available. There Taylor is less guarded: "If I was going to put a breakdown on it I would say 80 per cent is about us and what we are trying to achieve and do, the other 20 per cent depends on who we are maybe playing," he said.

"The majority is about what we are all about and want to achieve. We want to bring power to the game and guys who have repeatability. We need to have guys with a high work rate, so certainly those things are very important in terms of our rugby culture and the way we want to play. We are looking at players who can produce that and give us that."

So take those principles, assume Alex Dunbar won't make it – though he will be on standby – and that Cotter will go with 18 forwards and 13 backs, remember the need for spares to cover those bugs. Then see how close your squad is to Cotter's when it comes out early this afternoon.