Viruses are not made of cells, which is one reason scientists debate whether they are truly alive., A virus can only reproduce by entering a living cell and using that cell’s machinery to make more viruses., Viruses are extremely small and usually much smaller than bacteria, so they cannot be seen with a regular microscope., There are more viruses on Earth than stars in the observable universe., Not all viruses are harmful to humans; many infect only plants, animals, fungi, or bacteria., Viruses that infect bacteria are called bacteriophages, or simply phages., Some viruses can change quickly through mutation, which can make them harder to control., The common cold can be caused by many different viruses, including rhinoviruses and some coronaviruses., COVID-19 is caused by a coronavirus called SARS-CoV-2., Coronaviruses get their name from the crown-like spikes on their surface., SARS-CoV-2 mainly spreads through respiratory droplets and tiny airborne particles released when people breathe, talk, cough, or sneeze., Vaccines help the immune system recognize viruses faster and reduce the risk of severe illness., Viruses can survive for different lengths of time outside the body depending on the type of virus and the surface or environment., Some viruses, such as influenza viruses, have seasonal patterns and spread more at certain times of the year., HIV is a virus that attacks the immune system, especially certain white blood cells., Measles is one of the most contagious viral diseases known., Some viruses can remain dormant in the body and reactivate later, such as the virus that causes chickenpox and shingles., Scientists use genome sequencing to track how viruses like SARS-CoV-2 change over time., COVID vaccines were developed unusually quickly because of global cooperation, prior research on coronaviruses, and new vaccine technology., Simple public health actions such as handwashing, ventilation, staying home when sick, and vaccination can help reduce the spread of many viruses.

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