Why are reactive oxygen species (ROS) bad?

Reactive oxygen species (ROS) are highly reactive and can damage all macromolecules, including lipids, proteins, and nucleic acids. A balance in the ROS levels for homeostasis is required for a healthy body. Excessive amounts of ROS can cause oxidative stress, triggering cell apoptosis and leading to aging as well as pathologies ranging from autoimmune diseases to cardiomyopathies.