Influenza H1N1 is a relatively new illness, and is still being studied. The following is based on the latest preliminary information and may change as more is known.

Glossary of Terms
Pandemic Flu (Swine Flu): A glossary of terms for writers and editors, UK Govt Office for Science. Click to download.

Symptoms
Initial symptoms are similar to the symptoms of "regular" (seasonal) flu:  fever, body aches, headache, runny nose, sore throat and cough. Some individuals may have respiratory symptoms without fever. A number of people have also had vomiting and diarrhea.

Some infected people develop a severe illness, which can include pneumonia and difficulty breathing. This can happen within about 5 days. Although anyone who gets H1N1 might become severely ill, people in certain groups appear to be at higher risk.

Higher risk groups for severe disease include:

• Pregnant women
• People with chronic health conditions such as diabetes, asthma, other lung diseases, heart disease, obesity
• People with compromised immune function (e.g. HIV positive)

In addition, the US CDC considers the groups who are at higher risk for complicated seasonal flu to be at higher risk from influenza H1N1 at this time. In addition to the groups above, they are:

• Children < 5 years of age
• People > 65 years of age
• People < 18 years on long-term aspirin therapy
• Residents of nursing homes and other chronic-care facilities

Incubation period
The incubation period is the time between  the moment a person is infected with the H1N1 virus and the time they start showing symptoms. People in the incubation period appear healthy, but they can sometimes pass the virus on to others.

Right now, the incubation period of H1N1 appears to be somewhere between one and seven days. It is likely most infected people start having symptoms within four days.

Infectious period
The infectious period is the time when a person with H1N1 is contagious, which means they can pass the virus on to others. The infectious period for H1N1 is not yet completely known. The US CDC considers it

• Start one day before the person shows symptoms (gets sick)
• Last for seven days after the symptoms appear

If infected people are still sick after seven days, they should be considered potentially contagious until their symptoms have disappeared. Children may be contagious for longer, up to 10 days.

Early small studies (see here and here) show that almost all people on treatment are unlikely to be infectious after 5 days, and most are not infectious after 2.

Transmission
H1N1 flu spreads from person to person in the same way seasonal flu viruses spread: through infectious respiratory droplets. These droplets are expelled when an infected person coughs, sneezes or talks. These droplets can spread about 1 to 2 meters (3-6 feet) before they fall. If they get into a healthy person's nose or mouth, the person can get infected. (This is called "direct contact".)

People can also get infected through "indirect contact". The droplets expelled by a sick person can contaminate objects. Flu virus can live on hard surfaces for hours, as long as 2 days. It can get onto people's hands when they touch something contaminated, such as doorknobs, keyboards, counters, etc. If the person then touches his or her eyes/nose/mouth with their hand, the virus can enter their body.

Influenza may also spread via "aerosol" transmission, where the virus floats in the air and infects people. This is particularly possible in crowded places. This is not thought to be a frequent mode of flu transmission, and may only happen in unusual circumstances where ventilation is poor.

At this stage, the secretions and bodily fluids of someone with H1N1 are considered potentially infectious. This includes vomit, diarrhea and mucus. Investigators reported in March 2010 that the virus was undetectable in blood donated by infected people. There are no known cases in which someone was infected with pandemic flu via blood transfusion, or contact with a patient's blood.  

Neurological Complications were reported in July 2009 in Mortality and Morbidity Weekly Report. Four children from Texas who had pandemic H1N1 flu suffered neurologic complications (such as seizures, brain swelling, changes to their mental status including confusion and disorientation). Such complications sometimes occur with seasonal flus as well as other respiratory tract infections. All four children were given antiviral drugs and recovered with no lasting neurological symptoms.

In a report published in March 2010, the Journal of Clinical Infectious Diseases stated an incident of brain death from a patient infected with pandemic H1N1 flu. The child developed encephalopathy (a disorder of brain tissue) which subsequently led to brain death. Brain death occurs when there is irreversible damage to brain cells.

Cardiac complications have been reported with pandemic influenza. These rare complications can occur with any viral infection. But this potentially fatal occurence seems more frequent with the pandemic influenza virus. This was reported in February 2010 in Journal of the American college of Cardiology. Four children had severe form of myocarditis, the inflammation of heart muscle. One child died and other three survived with the help of life support systems.

Studies

Risk Groups
Infection and death from influenza A H1N1 virus in Mexic a retrospective analysis from the Lancet dated 19 Dec 2009