Infection is a condition that is caused by a multiplication of an infectious agent in the body. Hosts can fight infections using their immune system. Mammalian hosts react to infections with an innate response, often involving inflammation, followed by an adaptive response. Pediatric infectious diseases specialists treat a wide range of infectious and immunologic diseases such as those caused by bacteria, viruses, fungi, and parasites. Many organisms live in and on our bodies. They are normally harmless or even helpful, but under certain conditions, some organisms may cause disease. Infectious diseases are sometimes called contagious disease when they are easily transmitted by contact with an ill person or their secretions. Thus, a contagious disease is a subset of infectious disease that is especially infective or easily transmitted. Other types of infectious or transmissible diseases with more specialized routes of infection, such as vector transmission or sexual transmission, are usually not regarded as contagious, and often do not require medical isolation of victims. Some infectious diseases can be passed from child to child. Some are transmitted from insects or animals. In some cases, children are acquired by ingesting contaminated food or water or being exposed to organisms in the environment. Many infectious diseases, such as measles and chickenpox, can be prevented by vaccines. Frequent and thorough hand-washing also helps protect you from infectious diseases.