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.

美国联邦卫生局官员于周日透露，猪流感已经在美国急速传播，有30个州已发现226起病例，预计未来几天在其他州还会出现更多病例。

美国疾病控制预防中心科学与公共卫生部副主任Anne Schuchat 博士在一次新闻发布会上说，猪流感正蔓延至整个美国，现在这种病毒已经出现在美国大部分地区。值得庆幸的是，美国的大部分病例并不严重。墨西哥卫生局官员也声称病情形势似乎正在稳定下来。不过Schuchat 博士却认为美国猪流感仍处于危险期。

她解释说，这种病毒被科学家称为甲型H1N1流感病毒，有很多致癌的特性，一般出现在流感季节结束时。这种病毒以前未出现过，所以人们缺乏对它的预防能力。

不同于普通的季节性流感，猪流感在年轻人中的发病率很高。患者的平均年龄是17岁。

Schuchat 博士说，经证实病患很少有超过50岁的，他们大多比较年轻，至于这种病毒在未来几周能否得到有效控制则无法估计，不过可以肯定它不同于季节性流感。

她还说，以往最容易患流感的是老年人和婴幼儿，而这次美国因患猪流感住院治疗的30名患者大多是青少年。现在美国疾病控制预防中心正从病毒样本中提取培养种苗用作疫苗。

农业部兽医局首席兽医官John R. Clifford博士说，他们在几周前就已经开始研究制作疫苗防止猪感染甲型H1N1流感病毒，他说以前从未在这个国家的猪身上发现过这种病毒。但是加拿大的猪却从人身上感染这种病毒，说明动物也很容易被感染。

据世界卫生组织统计，在世界范围内已有18个国家新发现898起猪流感病例。

据西班牙卫生部部长公布，在美洲以外的地区，出现病患最多的是西班牙，有44起病案。除四名从墨西哥返回的游客，其他猪流感患者都已痊愈。他在周一还表示会加强对飞机场的控制，但并没有提及具体措施。来自疫区的乘客需要填写健康调查表，乘务人员要戴手套和口罩。

世界卫生组织周日在日内瓦召开视频会议，讨论H1N1流感病毒可能在人和猪之间相互传播的问题。加拿大官员在周六宣称，一名携带该病毒的农场工人将病毒传染给了亚伯达省的猪。

卫生组织食品安全专家Peter K. Ben Embarek说，伴随这种疾病可能会出现另一个问题，即在农场或屠宰厂和患病动物亲密接触的人会从猪身上感染病毒，并且人类也曾有因为与病猪接触而染上其他猪流感病毒的先例。同样的情况也会发生在这种猪流感的传播中，因此在未采取预防措施的情况下应该避免让人接触患病动物或让动物接触病人。

Ben Embarek博士和其他卫生部官员重申，适当食用煮熟的猪肉和熏制的肉类食品（比如火腿）不会感染病毒。

据调查发现，加拿大的猪流感是由亚伯达省几个月前颁布的一项政策发现的，这项政策规定农民和兽医须向政府上报所有猪流感病案。加拿大猪肉委员会主席Jurgen Preugschas却认为，由于国家针对H1N1流感病毒爆发已于4月24日作出加强对肉类工业监督的决定，所以卫生部可能会对亚伯达农场的病案快速做出反应。

而美国的农民无需向政府上报猪流感病例。 Clifford博士说美国只有某些地区才有猪流感，而且每年美国都要检查一些病猪和分析这种病毒，所以H1NI流感病毒在这个国家并不严重。

他还认为，人有流感季节，猪也一样，而且在秋冬季节容易得流感。他还说，现在美国已广泛使用疫苗，仅去年一年就生产了58,000,000支。如果H1N1疫苗有新品上市，个体养猪户可自行决定是否使用新疫苗。

纽约的研究员仍在调查皇后区的旧金山预科学校多名学生被传染的原因。其中58 起病案程度较轻，而且都与学校有关。

纽约市卫生部环境病预防中心副局长Nancy Clark表示，学校新建的通风系统并不是疾病产生的原因，因为调查发现学校的通风系统都很洁净通畅。