“…thus, leading to higher rates of HIV infection (Peacock & Slocum, 2004).
“According to Peacock and Slocum (2004), the higher rates of HIV …”
(Ivy, Dees, & Coe, 1999)
(Ivy et al., 1999)
Note: et is Latin and (not an abbreviation, no period)—al. is an abbreviation for the Latin alia (“others”) and therefore always uses a period. In the citation, a comma separates et al. from the date.