Almost always carries a negative flavor — pests, bacteria, mold, social ills (孳生腐败 'breed corruption'). For the neutral 'reproduce' use 繁殖 (fánzhí); for human population growth use 增长 (zēngzhǎng).
mistakes
Often written 滋生 in modern texts — they're interchangeable, with 滋生 now more common. 孳 is the older variant; you'll see it in classical and scientific writing.
