(CBS) As if they didn't have enough problems readjusting to life back at school after April's shooting, 23 Virginia Tech students were sickened by carbon monoxide at their off-campus apartment complex over the weekend. Two were reported in critical condition.

Carbon monoxide is an odorless, colorless gas that can cause sudden illness and death, by taking the place of oxygen in a person's bloodstream, starving tissues of the oxygen they need. The result is brain damage and degradation of other parts of the body and eventually death.

It's found all over American homes in things like cars, generators, lawn mowers, snow blowers, portable stoves, grills, and household appliances like gas ranges, furnaces and home heating systems.

Authorities believe the Virginia Tech students breathed in carbon monoxide that leaked into their building through a faulty valve on a gas-fired water heater.

"The fact that people can't see it or smell it makes it especially dangerous, because it can leave a person unconscious with no warning at all," The Early Show medical correspondent Dr. Emily Senay said.

Areas with proper ventilation are usually safe. It is when the gas builds up in an enclosed area that it becomes dangerous, she said.

There are symptoms to look for. People who are exposed to carbon monoxide may have trouble breathing, headaches, fatigue, nausea, chest pain or suffer from confusion. Sometimes the gas may overtake people so quickly that they don't even realize that something is happening until too late, Dr. Senay said.

"But if you do sense that this is happening to you, or to a friend or family member, you need to seek medical help immediately," she said. "According to the CDC (Centers for Disease Control and Prevention), nearly 500 Americans die from carbon monoxide poisoning in a typical year, and more than 15,000 need treatment at emergency rooms."

Dr. Senay said that people can protect themselves by installing a carbon monoxide detector in their home and making sure the batteries are fresh. She said the National Institutes of Health recommend that there is at least one detector for each level of the home. Install a detector near any gas burning appliance and have gas-burning appliances inspected regularly. Also, be sure to clean chimneys vents and flues at least once a year.

Even though it's only August, Dr. Senay says it is not too soon to take winter precautions.

"Some people try to use portable stoves, or charcoal grills and generators that are not designed for indoor use because of the carbon monoxide they give off," she said. "Every winter, people in this country die and become sick by trying to stay warm with these devices. While the weather is still warm, think ahead about things you can do in those situations, so you're not stuck in a house where the only sources of heat pose a carbon monoxide danger. It's not too soon to think about that at all."

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