The testimony of the nation’s top infectious disease expert countered President Trump’s upbeat assessment, describing a “mixed bag” of some bright spots amid worrying trends and unknowns.

Dr. Anthony S. Fauci told Congress on Tuesday that he was seeing a “disturbing surge” of infections in some parts of the country, as Americans ignore social distancing guidelines and states reopen without adequate plans for testing and tracing the contacts of those who get sick.

Dr. Fauci’s assessment, delivered during a lengthy hearing before the House Energy and Commerce Committee, painted a much grimmer picture of the coronavirus threat than the one given by President Trump, who claimed last week that the virus that had infected more than two million Americans and killed more than 121,000 would just “fade away.”

“The virus is not going to disappear,” said Dr. Fauci, the nation’s top infectious disease expert, who testified that the virus was not yet under control in the United States.