Infectious is not a good thing when you’re talking last winter’s strain of flu, but when the sounds of Chicago a cappella are infectious, it’s a virus you want to catch.

The nine singers, including some founding members, closed out their 15th anniversary season Saturday at Pilgrim Congregational Church in Oak Park with “Voces Latinas.” At intermission artistic director and founder Jonathan Miller astutely noted that he had just sprouted “happy feet.” The lively, throbbing Latin rhythms had faded out, but they were still pumping energy while he talked. The authentic music, from tango to samba, inspired a desire to ask for sangria (though none was served in the dark and reverent interior of this church).

In 1996, the group had been singing concerts for three years and launched their first subscription series, described by the Chicago Tribune’s John von Rhein as music exquisitely pure, beautifully balanced and nuanced, right up there with the best of them. Happily not much has changed. I can still write about the purity, balance, and nuance in the many sets of “Voces,” which Saturday included old, dancing, new, serious, and festive voices, and-what proved to be entirely serious rather than sappy-voices of love.

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Adding to the fine art that has marked Chicago a cappella since its inception was a catchy vitality, notably recreating the Latin rhythms that usually come through with maracas and congas, but which were replicated here with sighs, yelps, and jolts from voices and a little tasteful body percussion.

Think of Ritchie Valens’ back-up band in the early rock and roll “La Bamba” (which you might know from the 1980s movie of the same name). Then imagine those same rhythms coming through with clicks and hard consonants-you could hear drums and guitars, but it was all done here by some magical sleight of mouth. A running “da-choo” or “doom k-ch k-ch ta-koo” kept the beat in many of these songs, which were excellent, not only in these renditions, but in their uncannily clever arrangements.

Of the two works grouped under Modern Voices, “Siete Haiku” was especially stirring. The sounds of Jorge Córdoba Valencia’s settings were gorgeous, shimmering palettes of color and glitter that portrayed the subjects of seven haiku. The images seemed to hover in the air-delicate subjects like cherry blossoms and pine boughs, action-packed horses in gallop, and a frog springing into a pond. This music, though set to Japanese poetry, was unmistakably Hispanic and offered an arresting blend of the two cultures.

Unfortunately, the world premiere of “Estampas,” commissioned from the New York composer Tania León, a new work, setting poetry by favorite Cuban poets of León, did not measure up to the emotional or musical impact of nearly every other piece on the program.

What did ring true, however, was tenor Trevor Mitchell, who stole the show early on with his “Mata del ánima sola” (Tree of the lonely soul) by Antonio Estévez. The many other excellent solos of the night were strong and commendable, but Mitchell’s rich and creamy tone was soothing and so touching. He sang first of wistful longing in a soulful plaint and then suddenly dashed off in exuberant flights of a bounding, happy Venezuelan joropo, which, not knowing my ethnic dances, sounded like a bubbly salsa to me.

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After 15 years of consistently excellent choral singing, you would think this group might wear thin, or wear out. But a cappella has stretched its musical limits with challenging premieres, adventures in languages unknown in the mainstream, and the addition of music director Patrick Sinozich this year. The growing pains are at times slightly audible, but in Saturday’s program there wasn’t one rough edge. Even the brief moments of stand-up stories told by some of the singers, which in past shows have been a bit tedious, were polished and engaging.

From the driving “Salmo 150” of Ernani Aguiar, which offered 2000% of the minimum daily requirement of consonants at an exceptionally fast tempo, to the haunting aches of Pablo Casals’s “O vos omnes” (“see if there be any sorrow”), these voices exceeded even their own high standards of fine singing. When refined and vibrant culture at the level of Chicago a cappella is in the air, you might as well catch the fever.

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