Disseminate means to spread information, knowledge, opinions widely. Semin- derives from the Latin word for seed; the idea with disseminate is that information travels like seeds sown by a farmer.
Think about a teacher distributing a hand out at the beginning of a class. The dis- of disseminate and distribute come from the same Latin prefix which means "apart, in a different direction." But unlike papers distributed in class, information, once spread around in all directions, cannot be pulled back in. Think about false rumors or political smear campaigns and you'll understand that dissemination is usually a one-way process.