(S (S (BASENP (PRP He)) (VP (VBD noted) (SBAR (IN that) (S (BASENP (PRP it)) (VP (VP (VBD was) (ADJP (JJ difficult))) (S (VP (TO to) (VP (VB predict) (NP (BASENP (DT the) (NN activity)) (PP (IN of) (BASENP (DT the) (NN virus)))))))))))) (PERIOD .))