ew diseases are the source of great anxiety until scientists determine their causes and the way they are spread. The newest new disease, SARS, for severe acute respiratory syndrome, is a case in point.

Since its sudden appearance in Asia three weeks ago, SARS has spread to North America and Europe. But as health officials work to try to understand the new disease, the public has been left with unanswered questions.

Among other issues, many people are wondering how they can avoid getting SARS, how they will know if they have it, whether it can be cured and whether they should curtail travel to an affected area.

In some cases, there are answers to those questions and in many other cases there are not.

With only 69 reported suspect SARS cases in the United States, health officials say there is no reason for panic. For unknown reasons, many of the cases here so far have been milder than those reported in Asia. No deaths have been reported in the United States.

An estimated half-million Americans develop serious acute respiratory illnesses each year, but the cause of many of these is not found, in part because laboratory tests are often inconclusive. Outbreaks of influenza occur throughout the world every year. During a severe outbreak of influenza in an average year 36,000 Americans die.

But health officials do recommend precautions in travel. The Centers for Disease Control has urged people not to travel to affected areas unless they have essential business or personal affairs to conduct.

On Saturday, the centers broadened its cautions against traveling because of SARS to include all of mainland China, including Hong Kong; Hanoi, Vietnam; and Singapore.

Although Canada has reported 130 probable and suspected SARS cases, including six deaths, and Ontario on Monday suspended all nonurgent surgery and outpatient clinics in all of its 215 hospitals, the centers said it "is not advising against travel to or from Canada because there is not evidence of widespread community transmission" of SARS.

At this point, the main worries to health officials are the ability of SARS to quickly spread to large clusters of health workers caring for ill patients and the death rate, which is about 3.4 percent. That death rate is half that of West Nile fever, which was 6.7 percent last year, but is still considered relatively high by health officials.

Health officials initially issued a global alert, in part, because of fears that a new strain of influenza had appeared that might rival the magnitude of the 1918-19 pandemic that killed 20 million people worldwide. It now appears that SARS is not influenza.

From the start, World Health Organization officials have said that SARS generally requires sustained close face-to-face contact. But now its officials are investigating whether SARS can be spread through the air, water, sewage and contaminated objects. Until these and other issues can be settled scientifically — a process that may take weeks — health officials can only give advice and act on reasonable assumptions.

SARS is like any other acute respiratory infection; it starts with a fever of 100.4 degrees Fahrenheit or higher, malaise, headache, difficulty breathing and dry cough. Chest X-rays tend to show what doctors call "atypical pneumonia" in a lower lobe of a lung. In the next few days, the pneumonia often spreads to another lobe, but, for unknown reasons, rarely to the topmost lobes, as tuberculosis can do.

Then about five or six days later, the symptoms improve in about 80 to 90 percent of patients and worsen in the remainder. Many of the sickest patients need intensive care, even to the point of being connected to a mechanical respirator.

Why some people improve and others die is not known. So far, it appears that the most susceptible are those 40 years or older and those who had a chronic disease in the past. Those who have caught SARS range from children to the elderly.

Although little autopsy information is available, some findings so far indicate that victims die of progressive pneumonia.

Even the group that gets better may still require a lengthy hospital stay, in part because aside from regular nursing care and help in breathing there is no documented effective treatment for SARS. Recovery depends on the ability of the immune system to manufacture enough proteins known as antibodies and other immune factors.

Doctors do not yet know how long a patient can disperse the virus, even if symptoms have disappeared. In most cases, doctors are making only educated guesses as to when it is safe to let SARS patients leave the hospital. The proper use of barrier nursing techniques like masks, gowns, gloves and goggles have stopped the spread of SARS to the health workers caring for patients, although centers officials say they do not know if such measures are 100 percent effective.

Health officials do not know what causes SARS. The leading candidate is a previously unknown member of the coronavirus family, which can cause the common cold and severe diseases in animals. But other suspect viruses are in the picture and still others may emerge with further investigation, health officials say.