(S (S (S (ADVP (RB Usually)) (S (SBAR (WHADVP (WRB when)) (S (BASENP (EX there)) (VP (VBZ is) (NP (BASENP (DT a) (NN possibility)) (PP (IN of) (BASENP (NN rabies))))))) (COMMA ,) (S (BASENP (NNS doctors)) (VP (MD will) (VP (VB vaccinate) (BASENP (NNS patients))))))) (COLON :) (S (S (BASENP (DT the) (NN vaccine)) (VP (VBZ has) (NP (BASENP (DT a) (NN chance)) (SBAR (S (VP (TO to) (VP (VB act)))))))) (SBAR (IN before) (S (BASENP (DT the) (NNS symptoms)) (VP (VBP appear)))))) (PERIOD .))