Non-contagious infections, by contrast, usually require a special mode of transmission between persons or hosts. These include need for intermediate vector species (mosquitoes that carry malaria) or by non-casual transfer of bodily fluid (such as transfusions, needle sharing or sexual contact).[2] 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 infections are considered contagious, but only some of them are subject to medical isolation.