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There are two types of diseases:- a) congenital , found from birth( genetic disorders). b) acquired, that are developed after birth. Acquired diseases are result of infection due to exposure, exposure to pathogens. Influenza is caused by an rna virus called Myxovirus influenzae . It spreads through the air by droplet infection. The droplets emitted in to the air from sneeze, cough or spit of the infected person are laden with the virus. By inhaling the air containing these droplets one can get the infection. Common cold us caused by rhino virus . Droplets resulting from cough or sneeze of an infected person through contaminated objects like pens , books , cups, doorknobs, computer keyboard , mouse etc cause infection in a healthy person. Dengue fever and dengue hemorrhage fever are caused by dengue virus which belongs to genus Flavi virus . It is spread through Aedes mosquito . Avoiding exposure to mosquitoes is the only prevention. typhoid is caused by the bacteria salmonella typhi that infects the intestine of man. The bacteria enter the body through food and water contaminated by faecal matter pr excreta of a typhoid patient.