Sexual transmission is, as mentioned, not very efficient. There is a limit to the amount of sex and sexual partners a person can have. Their chances of having a HIV positive partner may increase as an epidemic spreads.But even if someone had hundreds of sexual partners a year, the risk per sex act is still low. In other words, most people will only transmit HIV a small number of times through heterosexual sex, if at all.
In contrast, if a hospital or clinic is giving injections with unsterilized equipment, the risk of infecting patients is many times higher than the risk of sexual transmission. Not only that, a single health practitioner can potentially infect many people per day, week or year. People infected through unsafe health care can also transmit HIV sexually, though they are no more likely to do so than those infected sexually. And every HIV positive person treated in a health facility that engages in unsafe practices can give rise to more HIV transmission. The potential for health care transmission is of a different order of magnitude than that for heterosexual transmission.