Although the C.D.C. advisory appeared to meet the definition of a Level 3 alert — “avoid nonessential travel” — it was issued as a Level 2.Levels 1 and 2 are “practice usual precautions” and “practice enhanced precautions.” A C.D.C. spokesman said it was kept at Level 2 because Level 3 indicates danger to all travelers while Level 2 means danger only to travelers with “specific risk factors” — in this case, pregnancy.