I also saw the warning message today. I installed a metamask Chrome browser extension a few days ago. I'm not doing anything else with Ethereum, so maybe that's the source of the warning?
Edit: response from Infura:
"Thanks for remaining vigilant! We investigated and believe this to be a false positive by @avast_antivirus. We have reported it and are waiting to hear back from their team. We will share any updates as we get them."
http://www.mediavida.com/foro/criptomonedas/infuraio-virus-falso-positivo-609572