A contagious disease is a subset category of transmissible diseases (can transmit from person to another) usually infections or some non-infection diseases, which are transmitted to other persons, either by physical contact (hence the name-origin) with the person suffering the disease, or by casual contact with their secretions or objects touched by them or airborne route among other routes. The non-contagious category of infections usually require a special mode of transmission between persons or hosts. These include need for intermediate vector species (mosquitoes that cause malaria) or by non-casual transfer of bodily fluid (such as transfusions, needle sharing or sexual contact). They can also be inherited from parents or caused by environmental or behavioral factors. The boundary between contagious and non-contagious infectious diseases is not perfectly drawn, as illustrated classically by tuberculosis, which is clearly transmissible from person to person, but was not classically considered a contagious disease. In the present day, most sexually transmitted diseases are considered contagious, but only some of them are subject to medical isolation.
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