There are various kind of viruses around us and there are viruses out there that are yet to be discovered. Every virus is different and behave very differently. Some viruses spreads aggressively, some viruses becomes deadly when it gets inside a human. A virus can become immune to medicine very fast and as a result that medicine will not work. Finding a new medicine can get challenging if this continues to happen. Virus has great ability to adapt and that what's creates the problem.