The FIFA virus is a term used to jokingly categorize the injuries and fatigue that football players suffer with after playing for their countries during the international break.
What is international break?
Each and every football club in the world will play their own league within the country. For example, England - Premier League, Spain - LaLiga , Italy - Serie A. This leagues matches will be playing for number of months. Within that period FIFA will organize or invite countries to play on specific tournaments like Euro Championship, COP Del America, AFC Championship. So all the leagues will be stoped for soemtime till the matches finish which is known as international break.
The FIFA virus is essentially a conflict of interest between football clubs and FIFA international competitions. Star players frequently travel great distances to play for their nations and upon returning to their clubs, they are often tired, jet-lagged and sluggish. From time to time, they come back injured.
The players affected by the 'FIFA virus' a lot of the times need to be rested prior to returning to their club duties.
It is common for large football clubs, like Barcelona, Juventus and Manchester United for example, to have most of their squads missing during the international break.
The rule is that if a player is injured for over 28 days due to playing internationally, FIFA is required to pay the portion of his/her club salary, until the player is fit again.
A good example of this is the back injury of Barcelona's Neymar Junior at the 2014 FIFA World Cup, after which FIFA compensated the club financially.