It depends on the virus: measles and chickenpox usually confer lasting immunity, while colds, flu, COVID, RSV, and norovirus reinfect you, because they mutate or because immunity fades fast.
The mutation trick (flu, COVID) changes the virus's surface so old immunity half-recognizes it; that is why flu shots update annually. The fading-immunity trick (RSV, common-cold coronaviruses, norovirus) lets protection drain within months, and norovirus adds strain specificity: a different strain can hit you right after the last one. The third pattern is dormancy, not reinfection: herpesviruses, chickenpox, and EBV stay in your body for life and can reactivate, which is what shingles is. Practical takeaways: vaccination after infection is still worthwhile because it broadens the immune response, a repeat flu is a new strain rather than the old one returning, and lingering fatigue after a virus is usually immune aftermath, not a fresh infection.
What to do
Common questions
Can you get the same virus twice?
It depends entirely on which virus, and the spectrum runs from "once, ever" to "every winter." The one-and-done group: measles, mumps, rubella, and chickenpox typically give durable lifelong immunity after infection (or vaccination), which is why getting measles twice is genuinely rare. The repeat-offender group works through two tricks. Trick one is mutation: influenza and SARS-CoV-2 change their surface proteins constantly, so last year's immunity only partially recognizes this year's strain, which is exactly why flu shots update annually. Trick two is fading or narrow immunity: common-cold coronaviruses and RSV produce immunity that wanes within months to a year or two, and norovirus immunity is both short-lived and strain-specific, so you can catch a different norovirus strain immediately after recovering from the last one. A few viruses do both tricks at once. Then there is the sleeper category: herpesviruses (cold sores, chickenpox, mono's EBV) never leave; they go dormant in your nerves or cells and can reactivate later, which is not a new infection but the old one waking up, as shingles painfully demonstrates. The practical implications: vaccination remains worth it even after infection (it broadens and strengthens the response), a "repeat" flu is usually a new strain rather than the old one returning, and persistent post-viral fatigue is more often immune aftermath than reinfection.