Whether or not infections follow a large scale natural disaster depends on the type of disaster, the timing of its occurrence, geographic characteristics of the stricken, and the social infrastructures in place. Moreover, prolonged reconstruction of the affected areas will increase the risk of further expanding the areas hit by endemic diseases as well as emerging or reemerging infections, causing international public health problems. The spread of an infection depends on the following elements in an intricate manner: 1) increase of a bioagent which becomes an infection source, 2) route of transmission changing both in quality and quantity as time elapses, and 3) changes in the sensitivity of individual hosts. Infections caused by bacteria/rickettsia/fungi include meningococcal infection, tuberculosis, cholera, shigellosis, typhoid fever, salmonellosis, plague, anthrax, melioidosis, leptospirosis, tetanus, coccidioidomycosis, typhus, and tsutsugamushi, etc. Those caused by viruses include hepatitis A, hepatitis E, measles, polio, rabies, dengue fever, Japanese encephalitis, West Nile fever, yellow fever, chikungunya fever, and hantavirus infection, etc. Those caused by protozoa include malaria, amoebic dysentery, cryptosporium, and giardia, etc. Infections after natural disasters fall into two categories, one resulting from the direct damage sustained in disasters and the other from migration and life in refugee camps. Soon after a major natural disaster strikes, the World Health Organization coordinates dialogues between public health authorities of the affected country and various assistance organizations regarding countermeasures against infections. Medical needs change at the scene of the disaster as time elapses, changing the type and scale of activities carried out by the various support organizations. In analyzing the epidemiological data from disasters, one must be careful in interpreting the voluntarily reported information. We need to consider the possibility of victim relocation, as well as the period and context of activities offered by various organizations,. Aid provided by experts from international organizations is essential to the disentanglement of the chaos that follows disasters, and cooperation among the organizations under normal circumstances is very important.