That's a good question. I'm not sure what temperatures you are getting, along with the rain, but the temperatures make more of a difference than rain.
The feathers of chickens help protect them from weather, but they only work well when dry. The biggest risk with chickens getting wet is the possibility of them getting too cold. Wet feathers cannot keep out the cold, nor help them maintain a good body temperature.
Wind is a big factor for chickens, wet or dry, but definitely worse, if their feathers are wet. Wind chill factors can make cold temperatures feel even colder, most especially with wet feathers.
A chicken wet from a warm summer rain will probably enjoy the bath, but in cold & windy weather a wet chicken is at risk of hypothermia. If their body temperature gets too low they are at risk of immune system failure, which can lead to illnesses.
It's important when building a chicken coop and yard to provide shelter from sun, wind & rain. The coop especially should be free of drafts and moisture, since night temperatures are much lower than day time.
Nice dry roosts are important, too, especially if your area experiences freezing temperatures. Most chicken breeds have bare legs and they need to cover them under their feathers at night to keep warm & regulate their body temperature.
I was called to rescue a flock of chickens living in a dark muddy pen in a barn. Most of the flock had lost toes to frost bite being forced to sleep on the damp ground with no roosts.
Sounds like you have a sick rooster and want to know why. If he got very wet & very cold, that could lead to his becoming sick. Best thing to do to help him through is keep him warm, out of drafts, and dry of course. Giving him vitamins would be good, either in his water or feed. If your area is experiencing cold temperatures, a heat lamp might be necessary.
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