1. Capable of causing infection: an infectious microorganism.
2. Capable of being transmitted by infection: an infectious disease.
3. Capable of transmitting a disease; contagious: Is the patient still infectious?
4. Spreading or tending to spread from one to another: an infectious laugh.
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2022 by HarperCollins Publishers. All rights reserved.
Indo-European & Semitic Roots Appendices
Thousands of entries in the dictionary include etymologies that trace their origins back to reconstructed proto-languages. You can obtain more information about these forms in our online appendices:
The Indo-European appendix covers nearly half of the Indo-European roots that have left their mark on English words. A more complete treatment of Indo-European roots and the English words derived from them is available in our Dictionary of Indo-European Roots.