When germs (e.g., bacteria or viruses) invade the body, they cause illness. Vaccines can prevent this from happening. Vaccines can be made from attenuated or inactivated germs or part of the germ structures, which help develop immunity by imitating an infection without causing disease. Once the imitation infection goes away, the body is left with specific memory cells which will remember how to fight the specific virus in the future. However, flu viruses can change over time and the human body sometimes fails to develop sufficient immunity. Under these circumstances, you can still fall sick despite being vaccinated against the virus. The development of improved vaccines including universal influenza vaccines that could provide protection against all subtypes of influenza viruses is a global effort.