Those feelings are a real thing called “sickness behavior,” which is sparked by the body’s response to infection. The same chemicals that tell the immune system to rush in and fend off invading viruses also tell us to slow down; skip the eating, drinking and sex; shun social interactions; and rest.

Viruses vary in their structure. A virus particle consists of DNA or RNA within a protective protein coat called a capsid. The shape of the capsid may vary from one type of virus to another. The capsid is made from the proteins that are encoded by viral genes within their genome.

Terms in this set (7)

• Virion. Mature infectious virus particle.
• Capsid. Protein shell that encloses and protects the viral nucleic acid.
• Capsomere. The morphological unit of the icosahedral capsid.
• Core.
• Nucleocapsid.
• Envelope.
• Spike proteins.

All viruses contain nucleic acid, either DNA or RNA (but not both), and a protein coat, which encases the nucleic acid. Some viruses are also enclosed by an envelope of fat and protein molecules. In its infective form, outside the cell, a virus particle is called a virion.

Viruses make us sick by killing cells or disrupting cell function. Our bodies often respond with fever (heat inactivates many viruses), the secretion of a chemical called interferon (which blocks viruses from reproducing), or by marshaling the immune system’s antibodies and other cells to target the invader.

Viruses make us sick by killing cells or disrupting cell function. Sometimes they kill cells and tissues outright. Sometimes they make toxins that can paralyze, destroy cells’ metabolic machinery, or precipitate a massive immune reaction that is itself toxic.

Antibiotics do not work on viruses, such as those that cause colds, flu, bronchitis, or runny noses, even if the mucus is thick, yellow, or green. Antibiotics are only needed for treating certain infections caused by bacteria, but even some bacterial infections get better without antibiotics.