Once the invasion takes place, the cell in essence is transformed into a factory that churns out hundreds and hundreds of copies of the virus, based on instructions encoded in its genetic material

But the human body as also other animals has evolved defense systems to protect against these kinds of infections. If you have a high dose of virus or if a virus has found ways to evade these protective measures, then this innate response can call in reinforcements

The work of these reinforcements to try to defeat the virus is typically what causes the symptoms of a viral infection — in other words, it's at this point when a person may come down with a fever and start to feel sick.

When the immune system is finally triggered, it can fight the virus.

The ability of a virus to evade detection is another reason it's difficult to treat with medications.

Antiviral drugs are also challenging to develop, because they need to work very specifically to combat certain viruses.

Because viruses hijack human cells, antiviral drugs can't readily target the same features without doing even more damage to the host. Nevertheless, points of difference between humans and viruses do exist, and their exploitation has led to some success.

Even when antiviral drugs are available for some viruses, they don't necessarily cure the infection. Antivirals do not generally kill viruses. Instead, they either inhibit the virus's ability to replicate or prevent it from infecting new cells. This means they can help the body eliminate the virus or manage chronic infections, but they don't necessarily destroy the virus entirely. 

Successful antivirals target and disrupt a process or structure unique to the virus, thereby preventing viral replication while minimising harm to the patient. The more dependent the virus is on the host cell, the fewer targets there are to hit with an antiviral. Unfortunately, most viruses offer few points of unique difference that can be targeted.

The resilience of viruses is what has made them such a menace throughout history, from flu pandemics to outbreaks of Ebola. And their ability to rapidly evolve, combined with the challenges of developing treatments and cures, will ensure that they remain a significant threat.

Our immune system can and does kill viruses (you can use the word kill here because inside a living body, the viruses are treated as living beings). It employs a multi-layered defense mechanism to identify, neutralize, and eliminate viruses from the body. This includes both innate and adaptive immune responses. 

The body has physical barriers like skin and mucous membranes that prevent viruses from entering. 

If a virus breaches these barriers, innate immune cells like phagocytes (e.g., macrophages) engulf and destroy the virus. 

Innate immunity also triggers inflammation, which helps recruit immune cells to the site of infection and creates an environment less hospitable to viruses. 

The adaptive immune system learns to recognize specific viral antigens (proteins on the virus surface). 

B cells produce antibodies that bind to and neutralize viruses, preventing them from infecting cells. 

T cells, including cytotoxic T cells, directly kill virus-infected cells, preventing the virus from replicating and spreading. 

The adaptive immune system creates memory cells, allowing for a faster and stronger response upon subsequent exposure to the same virus.

 The answer to your second question is - viruses are not eternal. While they can persist in certain conditions outside of a host, they are not considered alive and are eventually inactivated or destroyed. Viruses rely on host cells for replication, and their structures can be damaged or degraded, leading to their inactivation. 

Several things can disinfect viral particles, including certain chemical disinfectants like alcohol-based solutions, bleach, and some quaternary ammonium compounds. Additionally, heat, UV light, and even some detergents can be effective in disinfecting surfaces contaminated with viral particles. 

The soap literally dissolves the virus' structure! That's why handwashing is so important. It does not actually kill the virus, it deconstructs it, tearing it apart. This makes them unable to 'infect'.

Although formulations based on >70% ethanol are virucidal (3) ( substances that interact with and disrupt viral particles, rendering them unable to infect.), some viruses are resistant to alcohol (2).

So viruses are 'born', they evolve, get killed or denatured or inactivated and therefore, are not eternal!