CBSE Class 9 Answered

It is difficult to cure almost all diseases caused by viruses because of the following reasons:

• Viruses are not cellular. Therefore, it is not possible to kill them by simply disrupting their cells. Some viruses replicate inside human cells and then bud off from these cells inside an envelope made from the human cell membrane, which helps them evade the immune system on their way to infecting another human cell. 
• Many viruses are protected by protein capsids, which are extremely protective in nature.
• Antiviral drugs act on only a particular virus receptor. However, different kinds of viruses have different receptors. Hence, it becomes difficult to eliminate them.
• Most drugs available target a certain step of viral replication for a certain family of viruses. However, by the time the patient shows symptoms, the virus has already created countless copies of itself or becomes latent in human cells, and at that point it is too late for most of the antiviral drugs to be helpful since they target the replication itself.
• Many viruses do not spread in ways where they can be easily targeted.
• Viruses modify and adapt themselves rapidly to build up a resistance against the drug. 
• Viruses reproduce so rapidly that they have plenty of opportunity to change their genetic stripes with each new generation. This allows them to develop resistance to whatever drugs or vaccines which are used to eliminate them.