An infection is a contamination or colonization of pathogenic organisms in the human body which are capable of causing illness.
These are mostly bacteria, viruses, fungi or parasites. An infection does not neccessarily lead to a disease. A disease develops when pathological tissue reactions of organs occur, i.e. cystitis or meningitis, etc.
In severe and advanced cases this can lead to one or more organ failures. An example would be sepsis (blood poisoning). Such a life-threatening condition develops within a short time and is often accompanied by circulatory failure.