What causes pandemics?

Viruses and bacteria: Historically, pandemics are caused by viruses/bacteria that can easily spread. Some of the viruses/bacteria have been spread via human to human contact, and others via a vector such as mosquitos , fleas or animals. Circumstances that contribute to pandemic spread are overcrowding/population density, extensive travel/mobility, poor sanitation, widespread chronic disease and poor medical systems

Pandemic causes: A pandemic occurs when a serious infection/ disease is spread over many countries or multiple continents. It is caused by bacteria or viruses that are contagious & are spread amongst people (may or may not be direct person to person contact).