21.5 Viral Infections and Their Treatment

What is a Virus?

A virus is an acellular (non-cellular) infectious agent. Unlike bacteria, viruses are not living cells — they consist of:

• A nucleic acid core (either DNA or RNA, never both)
• A protein coat called a capsid
• Some viruses also have a lipid envelope derived from the host cell membrane

Because viruses lack their own cellular machinery (no ribosomes, no mitochondria, no cell wall), they are obligate intracellular parasites — they can only replicate inside a living host cell.

Why Are Viral Infections Difficult to Treat?

Treating viral infections presents unique challenges compared to bacterial infections:

Key challenge: Viruses replicate using the host cell's own machinery. Any drug that disrupts viral replication risks damaging the host cell too. This is the principle of selective toxicity — an effective antiviral must harm the virus without harming the host.

Antiviral Medications

Antiviral drugs work by targeting virus-specific steps in the viral life cycle:

1. Nucleoside Analogs

• Example: Acyclovir (used against Herpes Simplex Virus)
• Mechanism: Acyclovir mimics the nucleoside guanosine. It is selectively activated by a viral enzyme (thymidine kinase) and then inhibits viral DNA polymerase, blocking viral DNA synthesis.
• Because activation requires a viral enzyme, it has low toxicity to host cells.

2. Neuraminidase Inhibitors

• Example: Oseltamivir (Tamiflu) — used against Influenza
• Mechanism: Neuraminidase is a viral enzyme that allows newly formed virus particles to detach from the host cell surface and spread to new cells. Oseltamivir inhibits neuraminidase, trapping virus particles on the host cell and preventing further spread.

3. Reverse Transcriptase Inhibitors

• Example: Zidovudine (AZT) — used in HIV treatment
• Mechanism: HIV is a retrovirus that uses the enzyme reverse transcriptase to convert its RNA genome into DNA inside the host cell. Reverse transcriptase inhibitors block this step, preventing viral replication.
• This enzyme is unique to retroviruses, making it an excellent drug target.

4. Protease Inhibitors

• Example: Ritonavir — used in HIV treatment
• Mechanism: HIV protease cleaves large viral polyproteins into functional proteins needed to assemble new virus particles. Protease inhibitors block this cleavage, producing non-infectious viral particles.

5. Entry Inhibitors

• Mechanism: Prevent the virus from attaching to or fusing with the host cell membrane, blocking the very first step of infection.

Antiretroviral Therapy (ART) for HIV

HIV (Human Immunodeficiency Virus) attacks CD4+ T-cells, weakening the immune system and eventually leading to AIDS if untreated.

Antiretroviral Therapy (ART) uses a combination of drugs (typically 3 or more from different classes) to:

• Suppress HIV replication to undetectable levels
• Prevent the development of drug resistance
• Prevent progression to AIDS

This combination approach is called Highly Active Antiretroviral Therapy (HAART). Using multiple drugs simultaneously makes it much harder for the virus to mutate and develop resistance to all drugs at once.

Vaccines as Prevention

While antiviral drugs treat existing infections, vaccines prevent infection in the first place:

• Vaccines introduce weakened, killed, or subunit parts of a virus
• The immune system responds by producing antibodies and memory cells
• Upon future exposure to the real virus, the immune system responds rapidly

Examples: Influenza vaccine, COVID-19 vaccine, Hepatitis B vaccine

Challenges in Antiviral Drug Development

1. Selective toxicity is difficult to achieve since viruses use host machinery
2. Viral mutation leads to rapid development of drug resistance
3. Latency — some viruses (e.g., Herpes, HIV) can remain dormant in host cells, hidden from both the immune system and drugs
4. Diversity — each virus has a unique life cycle requiring specifically designed drugs