The United States experienced the beginnings of a pandemic of a novel strain of the influenza A/H1N1 virus, commonly referred to as "swine flu", in the spring of 2009. The earliest reported cases in the US began appearing in late March 2009 in California, then spreading to infect people in Texas, New York, and other states by mid-April. Early cases were associated with recent travel to Mexico; many were students who had traveled to Mexico for Spring Break. This spread continued across the country's population and by the end of May there were approximately 0 confirmed cases throughout all 50 states.

On April 28, 2009, the director of the Centers for Disease Control and Prevention (CDC) confirmed the first official US death of swine flu, a 23-month-old toddler from Mexico who died on April 27 while visiting Texas. By June 24, 132 deaths had been attributed to the virus. As of January 11, 2010, at least 554,000 deaths were attributed to the virus worldwide, and at least 12,469 deaths in the US were confirmed to be due to the virus. The CDC suspects, however, that the total number of deaths in the US is much higher than the official total, as some deaths probably went unconfirmed.

Alabama

On May 2, 2009, the Alabama Department of Public Health confirmed the first case of H1N1 (swine flu) in Madison, Alabama, one of two probable cases previously identified at an elementary school in Madison. As a result, all public schools and most private schools and daycare centers in Madison County, Alabama, including the cities of Huntsville and Madison, closed for two days. After meeting with the state public health officer on May 3, 2009, all Madison city elementary schools were to be closed through May 13, 2009, but were reopened after the CDC updated its guidance to schools.

As of December 5, 2009, the Alabama Department of Public Health reports 2453 confirmed and 36 deaths from swine flu. As of January 9, 2010, Alabama was the only US state reporting widespread A/H1N1 flu activity.

Alaska On May 10, 2009, the Alaska state Division of Public Health reported the state's first probable case of swine flu in a crew member of a Royal Caribbean cruise ship traveling in Alaska waters. On May 12, they confirmed that the woman had been infected with swine flu but they do not consider it to be Alaska's first case because she became ill before entering state waters.

As of December 5, 460 cases of swine flu and 11 deaths were confirmed in Alaska. On July 27, Alaska health officials confirmed the first H1N1-related death.

Arizona As of October 31, the Arizona Department of Health Services had reported a total of 6,302 confirmed cases of H1N1 infection since April 2009, with at least one case reported in each of the state's 15 counties. 786 of the cases had resulted in hospitalization of the patient, and 81 deaths were associated with H1N1 infection in that period of time. Of the H1N1 associated deaths reported in the state, 72% suffered an underlying medical condition (asthma, lung or heart conditions, cancer, metabolic disorders, pregnancy, immunosuppressive disorders, neurologic diseases or other chronic diseases) at the time of death.

On April 28, the first cases of H1N1 influenza infection in Arizona were confirmed in four school-aged patients in the Phoenix metropolitan area. The schools attended by the patients were immediately closed following recommendations from the CDC; after only 3 days, however, local authorities reopened the schools, stating that the H1N1 flu appeared to be no more deadly than the seasonal flu. School closures in the area have since been eliminated in favor of simply sending the infected children home, resulting in slightly elevated absence rates statewide. The first death in the state due to H1N1 swine flu complications was reported by the Maricopa County Department of Public Health on May 14. As of January 15, Arizona has reported 142 deaths due to confirmed A/H1N1 influenza, including at least 2 in 2010 alone.

Arkansas

During the 2009–2010 flu season, the Arkansas Department of Health reported 53 confirmed cases of swine flu. Four of the cases were from Camp Robinson, a US Army National Guard base in North Little Rock, Pulaski County.

California Deaths by county:

Alameda – 33 Berkeley City – 1 Butte – 3 Calaveras – 5 Contra Costa – 15 El Dorado – 1 Fresno – 22 Humboldt – 4 Imperial – 2 Kern – 22 Kings – 3 Lake – 1 Long Beach City – 7 Los Angeles – 149 Madera – 4 Marin – 5 Mendocino – 4 Merced – 5 Monterey – 4 Napa – 1 Nevada – 1 Orange – 56 Placer – 5 Riverside – 40 Sacramento – 22 San Benito – 1 San Bernardino – 43 San Diego – 63 San Francisco – 8 San Joaquin – 13 San Luis Obispo – 3 San Mateo – 11 Santa Barbara – 9 Santa Clara – 21 Santa Cruz – 3 Shasta – 2 Solano – 8 Sonoma – 11 Stanislaus – 13 Tehama – 2 Trinity – 1 Tulare – 6 Tuolumne – 1 Ventura – 14 Yolo – 6 Yuba – 3 Total – 657

The first two cases detected in the US were two children living in San Diego County and Imperial County who became ill on March 28 and 30 respectively. A CDC alert concerning these two isolated cases was reported in the media on April 21. As of April 24 eight human cases were known in the US, including six in Southern California. The patients have recovered. The acting director of the US Centers for Disease Control (CDC) said that preliminary tests on seven out of fourteen samples from patients in Mexico had matched the virus found in the US, which experts say is a new strain of swine flu. None of the US patients had any contact with pigs, leading CDC officials to believe that human-to-human transmission has been occurring.

Gov. Arnold Schwarzenegger and the state of California activated the Joint Emergency Operations Center of the Department of Public Health, and are coordinating with the California Emergency Management Agency, the CDC, and the Mexican government. They have additionally stepped up other preparations to lessen the flu's threat. On April 28, Gov. Schwarzenegger declared a state of emergency which allows the state to deploy additional resources to the Department of Public Health and more quickly and easily purchase equipment and materials.

St. Mels Catholic School in Fair Oaks was closed after Sacramento County Health Department notified the school that a 7th grade student who reportedly recently returned from a family vacation in Mexico tested positive for an unidentified strain of influenza virus A. On April 27, CDC officials confirmed that the student tested positive for swine flu. In Marin County, a grandmother and her 20-month-old granddaughter have been confirmed to have the flu. By April 28, the CDC had confirmed 10 cases of swine flu in California.

California State University, Long Beach reported on April 29 that a student had returned a "probable positive" test result for swine influenza. The student showed symptoms on Sunday April 26 and went to the campus health services office the following day. The test results were received by the school on April 28 and distributed to all students and faculty. The affected student had not attended any classes since falling ill and has an apparently mild case of the disease that does not appear life-threatening. Three high schools in Riverside County were closed April 29 after two teenage girls, from Corona and Indio respectively, were confirmed to have contracted the virus. Branham High School in San Jose was closed that day for a week after one teenage girl was confirmed to be a probable case. Rucker Elementary School in Gilroy is set to be closed on Friday, May 1 after at least one student was being tested after exhibiting flu-like symptoms. Three students at the Grizzly Youth Academy in San Luis Obispo are confirmed cases of swine influenza. As many as 73 students are exhibiting flu-like symptoms at The academy and are assumed to have the virus. All are being isolated. The academy is set to stay open. On May 3, 2009, parent of students attending King Middle School in Berkeley were notified that their school would be closed down due to a swine flu outbreak involving students at the school. Days later, other schools were closed down because of students or staff exhibiting flu-like symptoms; for example, Sunnyside Elementary School and Woodville Elementary School, both in Tulare County.

On June 1, 2009, the first two deaths were confirmed, one by a middle-aged man in San Bernardino County, and another one by a middle-aged woman in Los Angeles County. A third death was confirmed on June 4, when a nine-year-old girl from Contra Costa County died. at the Library of Congress (June 17, 2009). They were followed by an Orange County man's death on June 8 and a middle-aged man's death in Alameda County on June 9. As of June 9, there were 973 confirmed cases, 266 probable cases, and five deaths caused by the H1N1 flu. On June 10, another middle-aged man died in Alameda County, thus making it the third death in the San Francisco Bay Area.

On July 1, a woman who had been hospitalized in Marin County died of swine flu.

As of July 23, 2009, 2,655 cases and 61 deaths in California had been confirmed.

A list of hospitalizations and deaths can be viewed at the California Department of Health Website.

As of mid-September 2009, California had 2,655 cases and 152 deaths. As of January 15, at least 479 California residents had died from confirmed H1N1 infection.

Colorado On April 30 two cases of the flu virus were confirmed in the state of Colorado. The confirmed cases were a woman from Arapahoe County who recently returned from a cruise to Mexico and a Denver International Airport baggage handler. Two more H1N1 influenza cases were confirmed on May 2, both in Jefferson County, Colorado. One case is a middle school student, which has caused the school he attends to close for a week. The tally increased to seven on May 4 when The University of Colorado at Boulder (CU) confirmed three of its students contracted the virus.

There were 171 cases reported in Colorado as of July 18. On July 29, 2009, Colorado reported its first A(H1N1) swine flu death in El Paso County of a woman in her forties.

Connecticut On April 28, it was announced that there were suspected cases of swine flu in three Connecticut towns. Schools were closed due to suspected cases in East Haddam and Wethersfield, though tests on these patients later came back negative.H1N1 cases negative in two towns WTNH. Retrieved April 30, 2009. On April 30, two students at Fairfield University were announced as having "probable" swine flu, in addition to another person in Glastonbury, bringing the total number of likely cases to 6.Ffld U students may have swine flu Connecticut Post. 2009-04-30. Retrieved April 30, 2009.6th probable H1N1 case in Glastonbury WTNH. Retrieved April 30, 2009. On May 1, the first confirmed case of swine flu was reported in Connecticut in Stratford. On May 2, the second confirmed case was reported in a child from Middlefield that had recently returned from a family trip to Mexico. On May 5, Fairfield University announced that two "probable" cases had tested positive for the H1N1 flu, with five other "probable" cases awaiting test results. One May 8, Fairfield announced that those five students had also tested positive, although the students by that time were nearly recovered, and there remained one "probable" case remained to be confirmed. On Wednesday, June 3, The first death confirmed to be linked to the H1N1 virus happened in New Haven County. As of Wednesday, June 17 there are 7 confirmed cases of the H1N1 virus at Joseph A. Foran High School of Milford. The school is following a half day schedule until the end of the year, which was reported as being successful for preventing the virus.

By July 22, 2009, there were 1,713 confirmed cases of swine flu in Connecticut. New Haven has the most of any town with 175 cases.