( ( ( ( A ( sexually ( transmitted infection ) ) ) ( -LRB- ( STI -RRB- ) ) ) ( -LRB- ( ( also ( known ( as ( ( ( ( a ( sexually ( transmitted disease ) ) ) , ) or ) STD ) ) ) ) -RRB- ) ) ) ( ( is ( ( an infection ) ( caused ( by ( ( a pathogen ) ( that ( ( spreads mainly ) ( through ( sexual contact ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NP (NP (DT A) (RB sexually) (JJ transmitted) (NN infection)) (PRN (-LRB- -LRB-) (NP (NNP STI)) (-RRB- -RRB-))) (PRN (-LRB- -LRB-) (VP (ADVP (RB also)) (VBN known) (PP (IN as) (NP (NP (DT a) (RB sexually) (JJ transmitted) (NN disease)) (, ,) (CC or) (NP (NNP STD))))) (-RRB- -RRB-))) (VP (VBZ is) (NP (NP (DT an) (NN infection)) (VP (VBN caused) (PP (IN by) (NP (NP (DT a) (NN pathogen)) (SBAR (WHNP (WDT that)) (S (VP (VBZ spreads) (ADVP (RB mainly)) (PP (IN through) (NP (JJ sexual) (NN contact))))))))))) (. .)))
A sexually transmitted infection (STI) (also known  as a sexually transmitted disease, or STD) is an infection caused by a pathogen that spreads mainly  through sexual contact.
(ROOT (S (NP (NP (JJ Pathogens) (NN spread)) (PRN (-LRB- -LRB-) (PRP s) (-RRB- -RRB-))) (VP (VBZ stis) (PP (IN during) (NP (JJ sexual) (NN contact)))) (. .)))