Swine flu has become widespread in the United States, with 226 cases in 30 states and more expected to turn up in additional states in the next few days, federal health officials said Sunday.

“I think it’s circulating all over the U.S.,” Dr. Anne Schuchat, the interim deputy director for science and public health at the Centers for Disease Control and Prevention, said in a news conference. “The virus has arrived, I would say, in most of the country now.”

The good news, Dr. Schuchat said, is that most cases in the United States have been mild, and health officials in Mexico said that cases there seemed to be leveling off.

But Dr. Schuchat said, “I don’t think we’re out of the woods yet.”

She said the virus, called H1N1 by scientists, had a number of unusual features that were cause for concern. It has flared up at a time of year when the flu season is normally ending. It is new, so people probably have little or no resistance to it.

And unlike the common types of seasonal flu, it appears to infect an unusually high percentage of young people. The median age of patients is 17.

“Very few confirmed are over 50,” Dr. Schuchat said. “They tend to be younger. Whether it will pan out in the weeks ahead we don’t know, but it is a pattern that looks different from seasonal influenza.”

Of 30 people hospitalized in this country, she continued, a high proportion are older children and younger adults, a big change from the age groups that generally have the highest risk of needing hospital care for flu: the elderly and the very young.

She said scientists at the C.D.C. were preparing a “seed stock” from virus samples that could be used for a vaccine.

Similar work began several weeks ago to make a vaccine to protect pigs from the H1N1 virus, said Dr. John R. Clifford, the chief veterinary officer for animal health for the Department of Agriculture. He said the new virus had never been found in pigs in this country. But pigs in Canada have contracted it from a human, indicating that the animals are susceptible.

Worldwide, laboratories have now confirmed 898 human cases of the new swine flu in 18 countries, according to the World Health Organization.

Outside the Americas, the country with the most cases is Spain, with 44. All but four involved people who had recently traveled to Mexico, and all are recovering, the Spanish Health Ministry said.

The ministry said it would tighten controls at airports on Monday, but did not say how. Passengers arriving from affected areas have been filling out health questionnaires, and cabin crews have been supplied with gloves and masks.

The possibility that H1N1 might be passed back and forth between humans and pigs was discussed at a World Health Organization news teleconference from Geneva on Sunday. Canadian officials reported on Saturday that an infected farm worker had spread the virus to pigs in Alberta.

Peter K. Ben Embarek, a food safety scientist with the health organization, said there was also a risk that the disease could go the other way: that people who worked closely with sick animals on farms or in slaughterhouses could catch it from the pigs. In the past, he said, people have caught other types of swine flu from contact with infected pigs.

“Of course that could happen again here,” Dr. Ben Embarek said, adding that it was important to avoid exposing people to sick animals — and also to avoid exposing pigs to sick people.

Dr. Ben Embarek and other health officials reiterated that it was safe to eat properly cooked pork and also cured pork products like ham.

“You can continue to eat safely your prosciutto,” he said.

The cases in the pigs in Canada were detected because of a policy introduced months ago in Alberta, which requires farmers and veterinarians to report all cases of flu in pigs to the government. But Jurgen Preugschas, the chairman of the Canadian Pork Council, said the country’s decision on April 24 to increase oversight of the pork industry, in response to the H1N1 outbreak, probably led health officials to react quickly to the report from the Alberta farm.

In the United States, farmers are not required to report flu in pigs to the government. Dr. Clifford said that influenza was endemic in pigs in this country. Every year, he added, some pigs sick with the flu are tested and the virus is analyzed, and the new strain of H1N1 has never been found in pigs in this country.

“Just like people have flu seasons, so do pigs, normally in fall and winter,” Dr. Clifford said, adding that vaccine is widely used, and that some 58 million doses were produced last year for pigs in the United States. If a new vaccine is produced to prevent H1N1, it will be up to individual hog producers to decide whether to use it.

In New York, researchers are still trying to figure out why so many students at St. Francis Preparatory School in Queens became infected; 59 cases, all of them mild, have been linked to the school.

Nancy Clark, assistant commissioner for environmental disease prevention at the New York City Health Department, said one suspect, a new ventilation system at the school, did not appear to be a cause.

“We inspected many components of it, and generally found it to be in good condition and clean,” Ms. Clark said.

美國聯邦衛生局官員于周日透露，豬流感已經在美國急速傳播，有30個州已發現226起病例，預計未來幾天在其他州還會出現更多病例。

美國疾病控制預防中心科學與公共衛生部副主任Anne Schuchat 博士在一次新聞發布會上說，豬流感正蔓延至整個美國，現在這種病毒已經出現在美國大部分地區。值得慶幸的是，美國的大部分病例并不嚴重。墨西哥衛生局官員也聲稱病情形勢似乎正在穩定下來。不過Schuchat 博士卻認為美國豬流感仍處于危險期。

她解釋說，這種病毒被科學家稱為甲型H1N1流感病毒，有很多致癌的特性，一般出現在流感季節結束時。這種病毒以前未出現過，所以人們缺乏對它的預防能力。

不同于普通的季節性流感，豬流感在年輕人中的發病率很高。患者的平均年齡是17歲。

Schuchat 博士說，經證實病患很少有超過50歲的，他們大多比較年輕，至于這種病毒在未來幾周能否得到有效控制則無法估計，不過可以肯定它不同于季節性流感。

她還說，以往最容易患流感的是老年人和嬰幼兒，而這次美國因患豬流感住院治療的30名患者大多是青少年。現在美國疾病控制預防中心正從病毒樣本中提取培養種苗用作疫苗。

農業部獸醫局首席獸醫官John R. Clifford博士說，他們在幾周前就已經開始研究制作疫苗防止豬感染甲型H1N1流感病毒，他說以前從未在這個國家的豬身上發現過這種病毒。但是加拿大的豬卻從人身上感染這種病毒，說明動物也很容易被感染。

據世界衛生組織統計，在世界范圍內已有18個國家新發現898起豬流感病例。

據西班牙衛生部部長公布，在美洲以外的地區，出現病患最多的是西班牙，有44起病案。除四名從墨西哥返回的游客，其他豬流感患者都已痊愈。他在周一還表示會加強對飛機場的控制，但并沒有提及具體措施。來自疫區的乘客需要填寫健康調查表，乘務人員要戴手套和口罩。

世界衛生組織周日在日內瓦召開視頻會議，討論H1N1流感病毒可能在人和豬之間相互傳播的問題。加拿大官員在周六宣稱，一名攜帶該病毒的農場工人將病毒傳染給了亞伯達省的豬。

衛生組織食品安全專家Peter K. Ben Embarek說，伴隨這種疾病可能會出現另一個問題，即在農場或屠宰廠和患病動物親密接觸的人會從豬身上感染病毒，并且人類也曾有因為與病豬接觸而染上其他豬流感病毒的先例。同樣的情況也會發生在這種豬流感的傳播中，因此在未采取預防措施的情況下應該避免讓人接觸患病動物或讓動物接觸病人。

Ben Embarek博士和其他衛生部官員重申，適當食用煮熟的豬肉和熏制的肉類食品（比如火腿）不會感染病毒。

據調查發現，加拿大的豬流感是由亞伯達省幾個月前頒布的一項政策發現的，這項政策規定農民和獸醫須向政府上報所有豬流感病案。加拿大豬肉委員會主席Jurgen Preugschas卻認為，由于國家針對H1N1流感病毒爆發已于4月24日作出加強對肉類工業監督的決定，所以衛生部可能會對亞伯達農場的病案快速做出反應。

而美國的農民無需向政府上報豬流感病例。 Clifford博士說美國只有某些地區才有豬流感，而且每年美國都要檢查一些病豬和分析這種病毒，所以H1NI流感病毒在這個國家并不嚴重。

他還認為，人有流感季節，豬也一樣，而且在秋冬季節容易得流感。他還說，現在美國已廣泛使用疫苗，僅去年一年就生產了58,000,000支。如果H1N1疫苗有新品上市，個體養豬戶可自行決定是否使用新疫苗。

紐約的研究員仍在調查皇后區的舊金山預科學校多名學生被傳染的原因。其中58 起病案程度較輕，而且都與學校有關。

紐約市衛生部環境病預防中心副局長Nancy Clark表示，學校新建的通風系統并不是疾病產生的原因，因為調查發現學校的通風系統都很潔凈通暢。