The MMR vaccine protects against measles, mumps, and rubella (German measles). Many children in the U.S. are immunized as infants and toddlers, but that doesn't guarantee lifetime protection. And not everyone gets vaccinated as a child. Many adults move to the U.S. from countries without immunization programs.
When infected with Lycanthropy, or engaged in Beast Form as a result of Hircine's Ring or completing the Companions questline, persons become fully immune to vampirism. Argonians are naturally resistant to disease, and are 50% less likely than Men or Mer to contract vampirism. PC Disease resistance may fail to prevent vampirism. 2  It is also possible to contract at least one disease (Bone Break Fever) as a vampire, despite vampires being listed as having 100% disease resistance. 3  PC In some cases, becoming a vampire does not grant the Vampire's Sight ability.
You cannot encounter Cherrim in the wild, only it's previous stage: Cherubi. Cherubi is encountered 15% of the time via Honey Trees. It will appear from Level 5 to Level 15. Cherubi learns Sunny Day naturally at level 22, so you don't need a TM. Once Cherubi reaches Level 25 or higher, it can evolve into Negative Forme Cherrim. Use Sunny Day in battle and, voila, Positive Forme Cherrim appears. The form names will only appear in HeartGold/SoulSilver (probably), but they have been deemed as official canon.