Roseola is a mild, self-limiting viral illness caused by the human herpes virus type 6B or 7. It generally presents between 6 months and 3 years of age, with an estimated 80-90% of children having been affected during their first 2 years of life. It is droplet spread and is most infectious during the first few days (fever phase).
Initial symptoms include a high fever, runny nose and the child being generally out of sorts and tired. After 3-5 days of these initial symptoms resolve and a fine macular papular red/pink rash appears over the trunk. The face and limbs are rarely affected, though similar lesions may be seen on the soft palate and uvula. The rash is painless, not itchy, and does not blister. Lesions usually last around 2 days.
No treatment is required and complications are rare, but can include febrile convulsions.
References
https://dermnetnz.org/topics/roseola/
https://pemcincinnati.com/blog/briefs-the-fever-that-rashes/