An eruption is an explosion of steam and lava from a volcano. This word is also used for other explosions, such as "an eruption of emotions."
If there's an eruption of a volcano, you don't want to be anywhere near it. When a volcano erupts, it spews a huge amount of lava, ash, and steam into the air. If you were angry at a friend but held your tongue for a long time, then suddenly started yelling in anger, that's an eruption too. The sudden spreading of a disease could be called an eruption. Eruptions are quick, huge, explosive, and dangerous: they're never good news.