Key facts. Bacteria are single cells that can survive on their own, inside or outside the body. Viruses cause infections by entering and multiplying inside the host's healthy cells. It can be difficult to know what causes an infection, because viral and bacterial infections can cause similar symptoms.

All viruses have two basic parts: a protein coat that protects the virus and an inner core made of genetic material. A virus's genetic material contains the instructions for making new viruses. Like keys, a virus's proteins fit only into certain “locks,” or proteins, on the surface of a host's cells.

Bacteria are single cells that can multiply and cause infection. Viruses are much smaller than bacteria and need a host cell to reproduce. Viruses can only infect living cells, while bacteria can also infect non-living things like soil or water.

Knowing whether your sore throat is viral or bacterial is usually determined by symptoms. Viral sore throats usually consist of a cough, swelling in the throat, and runny nose whereas bacterial sore throats are typically accompanied with nausea and vomiting, stomach ache, and there is no cough.

Flu is caused by a virus that is spread from person to person. It causes seasonal epidemics each year.

Viruses are microscopic organisms that can infect hosts, like humans, plants or animals. They're a small piece of genetic information (DNA or RNA) inside of a protective shell (capsid). Some viruses also have an envelope. Viruses can't reproduce without a host.

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.