Yes. You can continue to feed BASIC or IMMUNO, you could possibly increase the dosage temporarily to support the horse while he is sick. The Equilin products aren’t medicines or a therapy, it’s always important to consult your vet in case of illness. An exception is when your horse has renal impairment.
Furthermore, the IMMUNO promotes the release of toxins, which is also what happens when a horse is sick - therefore this can be too much for the horse. When horses are very sick it is advisable to start feeding BASIC instead of IMMUNO.
It’s very important that in case of illness the horse keeps receiving enough vitamins and minerals to help him recover, so if you give less concentrates due to illness, you can add BALANCER or GROW.