Is rsv caused from thrush or vise versa?

No: Rsv is respiratory syncytial virus, which causes the common cold in older kids and adults. In younger children and babies, especially those under 2 years of age, RSV can cause bronchiolitis (think of it as a viral infection of the lungs with lots of mucus, coughing, and wheezing). It is not related to oral thrush.

They are not related: Rsv is a virus called respiratory syncytial virus which most commonly causes upper respiratory infections and pneumonitis (viral infection of lungs). It can be most serious in the very young or those who are immunocompromised because of a transplant. Thrush is caused by the yeast Candida Albicans and again it occurs most often in infants (normal). They are not related.

Immune system: The causes are certainly different. Thrush is typically causes by Candida and RSV is a viral infection. If your child has recurrent infections, or infections which do not resolve in the usual time frame, I would suggest an immune work up.