Simplified schematic of HIV transmission. After exposure to HIV through unprotected sexual contact, HIV has to pass several physiological barriers in the exposed individual (e.g. mucosal barrier) to reach a cellular compartment, where it can replicate in sufficient numbers to establish a systemic infection. However, not every exposure leads to infection. HIV prophylaxis with anti-HIV medication (e.g. PrEP) in the exposed individual decreases the replicative capacity of HIV and thus further decreases the probability of HIV infection. On the other hand, treatment of infected individuals (treatment-for-prevention) decreases their virus load and thus the number of transmitted virus, which can additionally decrease infection probabilities.