Which behavior is most likely to carry risk of serious infection

Using IV drugs

Explanation:

IV drugs are drugs injected directly into the user's vein. Because people who use this type of drug share the same needle and usually with no antiseptic attitude, this type of drug provides a high risk of infection. Contact with body fluids of a person with some kind of infection represents a direct contagion of this infection. Thus, as IV drug users use a single needle, they come into contact with each other's blood and spread the infection.