Influenza is caused by two main types of viruses – Influenza A and B, which change over time to evade defenses. Infection results primarily in respiratory symptoms, that can progress more seriously in immunocompromised or the elderly and cause death. Flu outbreaks are seasonal with more cases occurring in colder weather months. Illness spreads via respiratory droplets. Hard surfaces contaminated with droplets can pose risks, though this is not the main way flu spreads.