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Chronic infections, which can last for weeks, months, or a lifetime.
Latent infections, which may not cause symptoms at first but can reactivate over a period of months and years.
Most importantly, bacterial and viral infections, can cause mild, moderate, and severe diseases.
Throughout history, millions of people have died of diseases such as bubonic plague or the Black Death, which is caused by Yersinia pestis bacteria, 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 1.7 million people worldwide in 2011 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 medications.
The Differences Between Bacteria and Viruses
Although bacteria and viruses are both too small to be seen without a microscope, they're as different as giraffes and goldfish.
Bacteria are relatively complex, single-celled creatures with a rigid wall and a thin, rubbery membrane surrounding the fluid inside the cell. They can reproduce on their own. Fossilized records show that bacteria have existed for about 3.5 billion years, and bacteria can survive in different environments, including extreme heat and cold, radioactive waste, and the human body.
Most bacteria are harmless, and some actually help by digesting food, destroying disease-causing microbes, fighting cancer cells, and providing essential nutrients. Fewer than 1% of bacteria cause diseases in people.
Viruses are tinier: the largest of them are smaller than the smallest bacteria. All they have is a protein coat and a core of genetic material, either RNA or DNA. Unlike bacteria, viruses can't survive without a host. They can only reproduce by attaching themselves to cells. In most cases, they reprogram the cells to make new viruses until the cells burst and die. In other cases, they turn normal cells into malignant or cancerous cells.
Also unlike bacteria, most viruses do cause disease, and they're quite specific about the cells they attack. For example, certain viruses attacks cells in the liver, respiratory system, or blood. In some cases, viruses target bacteria.