Researchers have linked an infectious virus known to cause cancer in animals to chronic-fatigue syndrome, a major discovery for sufferers of the condition and one that concerned scientists for its potential public-health implications.

An estimated 17 million people world-wide suffer from chronic-fatigue syndrome, and the Centers for Disease Control and Prevention puts the U.S. figure at between one million and four million. CFS is characterized by debilitating fatigue and chronic pain, but there are no specific treatments, and the diagnosis is often made by ruling out other diseases. Thus there is disagreement in the medical community as to whether CFS is a distinct disease. A study showing a strong viral association with CFS could change that equation.

The significance of the finding, published Thursday in Science, extends far beyond the community of people living with CFS. Researchers are just as concerned about the finding that nearly 4% of healthy people used as controls in the study were also infected with the virus, called XMRV. If larger studies confirm these numbers, it could mean that as many as 10 million people in the U.S. and hundreds of millions of people around the world are infected with a virus that is already strongly associated with at least two diseases.

The study, the first to find live XMRV in humans, was done by researchers at the Whittemore Peterson Institute for Neuro-Immune Disease in Reno, Nev., the National Cancer Institute and the Cleveland Clinic.

In September, researchers at the University of Utah and Columbia University Medical Center found XMRV in 27% of the prostate-cancer samples they examined. That study also showed that 6% of the benign prostate samples had XMRV.

Neither study conclusively shows that XMRV causes chronic-fatigue syndrome or prostate cancer. But the National Cancer Institute was sufficiently concerned to convene a closed-door workshop in July to discuss the public-health implications of XMRV infection. 'NCI is responding like it did in the early days of HIV,' says Stuart Le Grice, head of the Center of Excellence in HIV/AIDS and cancer virology at NCI and one of the organizers of the July workshop.

Like HIV, XMRV is a retrovirus, meaning once someone is infected, the virus permanently remains in the body; either a person's immune system keeps it under control or drugs are needed to treat it. The virus creates an underlying immune deficiency, which might make people vulnerable to a range of diseases, said Judy Mikovits of the Whittemore Peterson Institute and one of the lead authors on the paper.

So far, XMRV, known fully as xenotropic murine leukemia virus-related virus, doesn't appear to replicate as quickly as HIV does. Scientists also don't know how XMRV is transmitted, but the infection was found in patients' blood samples, raising the possibility that it could be transmitted through blood or bodily fluids.

While Thursday's paper doesn't demonstrate conclusively that XMRV is a cause of CFS, additional unpublished data make it a very strong possibility. Dr. Mikovits said that using additional tests, the scientists determined that more than 95% of the patients in the study are either infected with live virus or are making antibodies that show their immune systems mounted an attack against XMRV and now had the virus under control. 'Just like you cannot have AIDS without HIV, I believe you won't be able to find a case of chronic-fatigue syndrome without XMRV,' Dr. Mikovits said.

At the July workshop, Dr. Mikovits also presented preliminary data showing that 20 patients of the 101 in the study have lymphoma, a rare form of cancer. The link between XMRV and lymphoma is still being investigated, but it raised the possibility that XMRV may be associated with other cancers in addition to prostate cancer. NCI's Dr. Le Grice said studies will be launched to determine whether XMRV is associated with other diseases. At the Whittemore Peterson Institute, Dr. Mikovits said they also found XMRV in people with autism, atypical multiple sclerosis and fibromyalgia.

研究人员将一种已知会导致动物癌症的传染病毒与慢性疲劳综合症联系起来，这对于慢性疲劳综合症患者来说是一个重大发现，同时科学家们也担忧其潜在的公共卫生问题。

预计全世界有1,700万人患有慢性疲劳综合症，美国疾病控制与预防中心（Centers for Disease Control and Prevention）预计美国患此症的人数为100万-400万。慢性疲劳综合症的特点是感觉衰弱疲劳并伴有长期疼痛，但没有有效的治疗方法，其确诊通常也是通过排除其他疾病。因此医学界对于慢性疲劳综合症是否为一种确定的疾病尚存争论。一项研究表明慢性疲劳综合症与一种强有力的病毒有关，这可能改变争论的形势。

周四在《科学》（Science）杂志上发布的这项研究结果意义重大，远远超出了患慢性疲劳综合症的人群。研究结果发现，研究中作为对照组的健康人中有近4%也感染了这种名为XMRV的病毒，研究人员对此担心不已。如果更大规模的研究证实了这些数据，那可能意味着美国最多有1,000万人感染了这种病毒，全球感染人数可能达数亿。这种病毒已经与至少两种疾病存在强烈关联。

这项研究由内华达州里诺市的Whittemore Peterson神经免疫疾病研究院、美国国立癌症研究院（National Cancer Institute）以及克利夫兰门诊医院（Cleveland Clinic）的研究人员进行。这是首次在人类身上发现活体XMRV病毒。

9月，犹他大学（University of Utah）和哥伦比亚大学医学院（Columbia University Medical Center）的研究人员发现，他们所检测的前列腺癌样本有27%含有XMRV病毒。该研究还发现，良性前列腺样本中有6%含有XMRV病毒。

上述两项研究都未能决定性地显示XMRV导致了慢性疲劳综合症或前列腺癌。但美国国立癌症研究院十分担忧，于7月召开了一个非公开的讨论会，讨论XMRV感染对公共卫生的影响。美国国立癌症研究院艾滋病和肿瘤病毒研究中心负责人格莱斯（Stuart Le Grice）说，美国国立癌症研究院对此采取了与艾滋病毒（HIV）发现之初一样的反应。格莱斯也是7月讨论会的组织者之一。

与艾滋病毒一样，XMRV也是一种反转录病毒，这意味着一旦感染，病毒就永久留在感染者体内；要么人体免疫系统对其进行控制，要么需要进行药物治疗。Whittemore Peterson研究院的米柯维茨（Judy Mikovits）说，这种病毒会导致潜在的免疫缺乏，这可能令人容易罹患多种疾病。米柯维茨是该论文的主要作者之一。

迄今为止，全称异嗜性小鼠白血病病毒相关病毒（Xenotropic murine leukemia virus-related virus）的XMRV病毒的复制速度似乎不像艾滋病毒那么快。科学家们也不知道XMRV通过何种途径传播，但患者的血液样本发现感染，令这种病毒通过血液或体液传染的可能性增加。

虽然周四发表的论文并未决定性地显示出XMRV病毒是慢性疲劳综合症的起因，但从未公布的其他数据看，这种可能性很高。米柯维茨说，通过另外的检测，科学家们确定研究中逾95%的患者受到活体病毒感染，或是产生了抗体，这表明他们的免疫系统向XMRV发起过攻击，现在已控制住这种病毒。米柯维茨说，就跟没感染艾滋病毒就不会患艾滋病一样，我认为不可能找到没有感染XMRV而罹患慢性疲劳综合症的病例。

在7月的讨论会上，米柯维茨还提交了初步数据，显示101个研究对象中有20名患者有淋巴瘤，这是一种罕见的肿瘤。XMRV和淋巴瘤之间的关系仍在研究，但这增加了XMRV与前列腺癌之外的其他癌症相关的可能性。美国国立癌症研究院的格莱斯说，将进行相关研究，确定XMRV是否与其他疾病相关。Whittemore Peterson研究院的米柯维茨说，他们还发现患孤独症、不典型多发性硬化和纤维肌痛的人体内也有XMRV病毒。