The proper process in virology that shows a virus causes a disease is as follows:
- First you have to believe a virus causes a disease.
- Take an appropriate sample (swab, sputum, blood, tissue, etc) from people who have similar symptoms of the disease.
- Purify (find) only the virus from the sample, and characterise (describe) it’s qualities.
- Do a control experiment by making sure samples from well people don’t contain the new virus.
- Make sure the new virus has come from outside the body.
- Expose well animals or people to only the new virus, and if they show the same or similar symptoms of unwellness as those the samples were taken from, then it can be said that the virus causes the symptoms of the disease.
- If well animals or people show the same symptoms then double check everything using the process above to find only the same virus from these animals or people.
- If you can’t get through all these steps then your belief that a virus causes a disease is inaccurate.
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A more scientific explanation and easy to understand explantion from professionals in their field about how to know a virus causes a disease.