If a contagious Ryugotchi on the same page as yours, then there's a chance your Ryugotchi will be infected when that Ryugotchi's owner visits this thread.
He looked normal? Are you sure that you weren't watching a feeding or clean up animation?
Your Ryugotchi's health doesn't decrease more quickly when it's sick, but there's a small chance that luck can deplete faster, and there's also a chance the evolutions might take longer to reach. On the plus side, an evolution instantly heals your Ryugotchi from any sickness!