The environment consists of numerous pathogens, which are agents, usually microorganisms, that cause diseases in their hosts. A host is the organism that is invaded and often harmed by a pathogen. Pathogens include bacteria, protists, fungi and other infectious organisms. We are constantly exposed to pathogens in food and water, on surfaces, and in the air. Mammalian immune systems evolved for protection from such pathogens; they are composed of an extremely diverse array of specialized cells and soluble molecules that coordinate a rapid and flexible defense system capable of providing protection from a majority of these disease agents.
Vaccines were developed to reduce the chance of infection of a particular disease, such as measles, mumps, polio, or chicken pox, by assisting the body to develop immunity. However, many diseases still do not have a vaccine, such as the deadly disease caused by the Ebola virus. Data from the World Health Organization indicates that more than 11,000 people died out of over 27,000 cases reported during the 2014–2015 outbreak. Though the majority of the cases were in Africa, Ebola did spread to other countries and prompted researchers to try to find a treatment. You can read more about this research at the Science Daily website.