SUNDAY, Aug. 22 (HealthDay News) -- If you've been sneezing your way through August, ragweed may be the culprit.

Ragweed season usually starts around mid-August and tends to torment allergy sufferers until the first frost sets in. Because ragweed counts are the highest between 5 a.m. and 10 a.m. on hot, dry and windy days, consider avoiding outdoor activities during that time, says an expert from the Saint Louis University Medical Center.

Seasonal allergy triggers abound, and knowing which ones affect you can help you avoid them to reduce the risk of annoying symptoms such as sneezing, stuffy nose and itchy eyes, Dr. James Temprano, assistant professor of internal medicine at the university, said in a university news release.

If you're not sure what's causing the problem, consider a skin test to determine which allergens are affecting you, Temprano suggested. For these tests, he said, doctors place small amounts of various allergens on or below the surface of your skin and watch for any reactions.

Once your seasonal allergies (such as ragweed, pollen and grass) are pinpointed, you'll know when you're most likely to experience symptoms and need to take preventive measures, such as keeping your windows closed.

Temprano offered these additional tips for other seasonal allergies:

• Grass allergies usually begin in May or June (but can appear earlier) and last most of the summer. Keep your windows shut in order to minimize your exposure.
• In the winter, when indoor allergies caused by dust mites and pet dander emerge, wash bedding weekly in hot water and dry using a high heat setting.
• If you are plagued by tree pollen allergies -- which typically begin in late February or early March and continue through May -- change your clothes and wash your hair after spending time outdoors.

Temprano added that if prevention doesn't work, you may want to talk with your doctor about allergy medications or immunotherapy, also known as allergy shots.

More information

The American Academy of Allergy, Asthma and Immunology has more about allergic reactions  .

THURSDAY, Aug. 5 (HealthDay News) -- The 2009 H1N1 swine flu virus used a new biochemical trick to hijack host cells, a feat that triggered the recent pandemic, according to an international team of scientists.

"We have found why the pandemic H1N1 virus replicated so well in humans," Yoshihiro Kawaoka, a leading influenza expert and a professor of pathobiological sciences at the University of Wisconsin-Madison's School of Veterinary Medicine, said in a university news release.

The H1N1 virus is a combination of four different avian and swine flu viruses that emerged over the past 90 years. It also includes genetic residue of the 1918 pandemic virus that killed as many as 20 million people, Kawaoka explained.

A typical flu virus requires the presence of two amino acids -- lysine and asparagines -- in specific sites on a key avian protein in order to jump from an animal and replicate efficiently in human cells.

But Kawaoka and colleagues found that the lysine amino acid is located in a completely different location on the avian protein in the H1N1 virus. This is what gives the virus the ability to adapt to and co-opt human cells.

The study is published in the Aug. 5 issue of PLoS Pathogens.

"This pandemic H1N1 has this mutation and is why it can replicate so well in humans. This gives us another marker to help predict the possibility of future flu pandemics," Kawaoka said.

As of July 25, 2010, the pandemic virus had caused more than 18,398 deaths worldwide, according to the World Health Organization.

More information

The U.S. Food and Drug Administration has more about the 2009 H1N1 swine flu.