Am into season 6 in my game and after my horse won the 1m classic trial then a 1m1 listed race (only entered cos a horse i bred and sold was favourite

) 4 days later i noticed in the top right hand corner a very small box, which you can only just see which says 'stable virus' i've been into adjust training and clicked every horse, all are fit and ready to race or a little jaded, clicked on each horse and nothing and no injuries showing..... What can i do?? Do i just withdraw my horses from their engagements until the box goes or is there something i can do?
Jon