Bacterial and viral infections have many similarities. Both types of infections are caused by microbes -- bacteria and viruses, respectively -- and spread by routes such as:

• Coughing and sneezing.
• Contact with contaminated people, especially through activities such as kissing and sexual intercourse.
• Contact with contaminated surfaces, food, and water.
• Contact with contaminated creatures, including household pets, livestock, and insects such as fleas and ticks.

Microbes can also cause:

• Acute infections,which are short-lived.
• Chronic infections,which can last for weeks, months, or a lifetime.
• Latent infections, which may not initially cause symptoms but which can reactivate over a period of months and years.
• Most importantly, bacterial and viral infections -- which are defined as an invasion of microbes able to reproduce in the body -- can cause mild, moderate, and severe disease.

Throughout history, millions of people have succumbed to diseases such as bubonic plague or the Black Death, which is caused by the Yersinia pestis bacterium, and smallpox, which is caused by the variola virus. In recent times, viral infections have been responsible for two major pandemics: the 1918-1919 “Spanish flu” epidemic that killed 20-40 million people, and the ongoing HIV/AIDS epidemic that killed an estimated 2 million people worldwide in 2008 alone.

Bacterial and viral infections can cause similar symptoms such as coughing and sneezing, fever, inflammation, vomiting, diarrhea, fatigue, and cramping -- all of which are ways the immune system tries to rid the body of infectious organisms. But bacterial and viral infections are dissimilar in many other important respects, most of them due to the organisms' structural differences and the way they respond to currently available antimicrobial medications.

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