Pediatric febrile seizures, which represent the most common childhood seizure disorder, exist only in association with an elevated temperature. Evidence suggests, however, that they have little connection with cognitive function, so the prognosis for normal neurologic function is excellent in children with febrile seizures. 
Epidemiologic studies have led to the division of febrile seizures into 3 groups, as follows:
Simple febrile seizures
Complex febrile seizures
Symptomatic febrile seizures
Essential update: Starting MMR/MMRV vaccination earlier may reduce seizure risk
In a case-series analysis of a cohort of 323,247 US children born from 2004 to 2008, Hambidge et al found that delaying the first dose of measles-mumps-rubella (MMR) or measles-mumps-rubella-varicella (MMRV) vaccine beyond the age of 15 months may more than double the risk of postvaccination seizures in the second year of life. [2, 3]
In infants, there was no association between vaccination timing and postvaccination seizures.  In the second year of life, however, the incident rate ratio (IRR) for seizures within 7-10 days was 2.65 (95% confidence interval [CI], 1.99-3.55) after first MMR doses at 12-15 months of age, compared with 6.53 (95% CI, 3.15-13.53) after first MMR doses at 16-23 months. For the MMRV vaccine, the IRR for seizures was 4.95 (95% CI, 3.68-6.66) after first doses at 12-15 months, compared with 9.80 (95% CI, 4.35-22.06) for first doses at 16-23 months.