Viruses have a structure that can only be seen under a microscope, and they are tiny—no bigger than the head of a pin. They have either RNA or DNA as their genetic material, and they lack cell walls. Viruses cannot reproduce on their own, so they enter host cells to copy themselves. In the process, they take over the host cell’s cellular machinery and force it to produce new viruses for them.
Unlike bacteria and plants, viruses do not generate energy from the food they eat. They are obligate intracellular parasites, and they obtain all of their energy from the host cell. During replication, viruses reroute or modify the host cell’s metabolic pathways so that they can make proteins and other viral components themselves. Viruses are also unable to make adenosine triphosphate (ATP), so they must borrow the host’s ribosomal machinery to synthesize ATP.
Viruses can be spread by touching surfaces or objects that have been touched by someone with the virus. They can also be passed from person to person through vaginal, oral or anal sex, a cough or sneeze, or by a mosquito, tick, or rodent bite. People can get infected with a virus through these methods and then become carriers, spreading the disease to others. Viruses can cause illnesses including the flu, herpes, Ebola, shingles and Zika. Viruses come in all shapes and sizes, and they can be found everywhere, even in your body. In fact, biologists estimate that trillions of viruses could be living inside you right now. But not all viruses make you sick. Most don’t even infect humans, and the ones that do usually don’t make you sick enough to visit your healthcare provider.