Ok so when it rains our enviroment temperature drops and it becomes much colder. I searched a little and discovered that when our surroundings are colder our nose temperature can drop until 5 degrees Celsius. Our immune system uses external vesicles (EV) to trap the bacterias and virus that cause constipation, flu, pneumonia and etc... and are expeled through mucus. Some of the EV can't survive when our nose is colder, droping our immune response by 50%. The bacterias and virus are around all year that's why when we are in fall and winter, the seasons where our enviroment is colder, people start to get sick. In conclusion, characters get sick after getting caught in the rain because their immune system response is compromised. I hope I answered your question!!! ☺️