An infectious disease, such as HIV and other STIs, that can be passed on to others. The route (method) of transmission varies among different types of communicable diseases – not all communicable diseases are passed on through unsafe sex. For example, HPV can be acquired by skin contact alone, and tuberculosis (TB) can be acquired from respirable air (i.e. breathing in the breath of someone with active TB). Some communicable diseases may be acquired by more than one route – for example HCV is commonly acquired by sharing drug injecting equipment and needles, but it can also be acquired (more rarely) by unsafe sex – particularly where there is blood-to-blood exposure between individuals.