HIV, Retroviruses & Respiratory Viruses: Influenza, Corona

Retroviruses are RNA viruses (family Retroviridae) carrying reverse transcriptase, an enzyme that copies their RNA into DNA inside the host cell. This DNA then inserts into the cell's own chromosomes, becoming permanent. HIV, the cause of AIDS, is the most important retrovirus in medicine and belongs to the lentivirus group.

These viruses matter because HIV silently destroys immunity over years, while respiratory viruses like influenza and coronaviruses spread fast through the air, causing seasonal epidemics and pandemics. Their biology drives antiretroviral drugs, annual vaccines and outbreak response.

Key Concepts

• Reverse transcriptase: converts single-stranded viral RNA into double-stranded DNA, reversing the normal flow of genetic information.
• Proviral DNA and integrase: the enzyme integrase stitches viral DNA into host chromosomes, where it can hide silently for years (latency).
• gp120/gp41 and CD4: HIV's spikes attach to the CD4 receptor plus a co-receptor (CCR5 or CXCR4) on helper T cells, which is why HIV kills these immune coordinators.
• Antigenic drift vs shift: drift is slow mutation of influenza's H and N surface proteins causing yearly flu; shift is sudden segment mixing that can start pandemics.
• Coronavirus spike and ACE2: coronaviruses are positive-sense RNA viruses whose spikes bind the ACE2 receptor on airway cells.

How It Works

HIV's gp120 locks onto CD4 and a co-receptor, the viral core enters, reverse transcriptase makes DNA from RNA, integrase inserts it into chromosomes, and the cell builds new viruses that bud off and mature via protease. Drugs block each step: NRTIs stop reverse transcriptase, integrase inhibitors stop integration, protease inhibitors stop maturation. Influenza's eight RNA segments copy themselves in airway cells while changing H and N proteins escape last year's immunity. Coronaviruses attach by spike protein, release RNA directly into the cell, and spread through droplets.

Worked Example

In a hostel, 300 students were exposed to influenza and 45 fell ill. The attack rate is:

1. New cases = 45; exposed = 300
2. Attack rate = (45 ÷ 300) × 100
3. 45 ÷ 300 = 0.15; 0.15 × 100 = 15%

Health teams use this to judge spread speed and whether isolation measures are working.

Common Misconceptions

• "HIV spreads by touch or mosquitoes." It spreads only through blood, unprotected sex, shared needles, and mother-to-child transmission. Casual contact is safe.
• "Flu is just a bad cold." Influenza causes high fever and exhaustion and kills hundreds of thousands yearly, mostly the elderly and chronically ill.
• "Antibiotics cure viral infections." They kill bacteria, not viruses. Using them for flu or COVID-19 only fuels resistance.

Real-World Applications

• Antiretroviral therapy (ART): drug combinations suppress HIV for life, making AIDS a manageable chronic condition.
• PrEP and PEP: protect at-risk people and healthcare workers after needle-stick injuries.
• Annual flu vaccines: reformulated yearly to match circulating H and N strains.
• Rapid diagnostics: HIV ELISA/rapid tests and RT-PCR for flu and coronaviruses enable early detection and isolation.

Retroviruses like HIV permanently insert their code into our cells; respiratory viruses like influenza and coronaviruses attack airways and keep changing their surface proteins. Drugs blocking reverse transcriptase, integrase and protease control HIV, while yearly vaccines and rapid testing control respiratory viruses.