Biology · Biological Classification · NEET
NCERT names these human viral diseases: mumps, smallpox, herpes, influenza (flu), common cold, and AIDS. Common cold is caused by rhino viruses. Just remember the group is VIRAL (not bacterial). Dysentery, plague, diphtheria, tetanus, cholera, typhoid and pneumonia are BACTERIAL — NEET loves to mix these two lists in match-the-column questions.
Most human viruses spread through air. When an infected person coughs or sneezes, tiny droplets (aerosols) carrying the virus go into the air, and a healthy person breathes them in. Examples: common cold, influenza and mumps spread this way. Some viruses spread by direct contact (herpes) or through body fluids like blood (AIDS/HIV). So the main routes are: air droplets, direct contact, and body fluids.
The common cold is caused by a VIRUS, not bacteria. The group of viruses is called rhino viruses. They infect the nose and respiratory passage but NOT the lungs. Symptoms include a blocked or running nose, sore throat, cough and headache. This is a very common trap: pneumonia (bacterial, Haemophilus/Streptococcus) attacks the lungs, but the cold virus stays in the upper airway only.
The most famous plant virus is the Tobacco Mosaic Virus (TMV), which causes mosaic disease of tobacco (studied by Ivanowsky in 1892). Symptoms in plants: a mosaic pattern of light and dark green patches, leaf rolling and curling, yellowing, vein clearing, dwarfing and stunted growth. Key NEET point: plant viruses usually have single-stranded RNA, while animal viruses have single or double-stranded RNA or double-stranded DNA.
A bacteriophage is a virus that infects bacteria (not humans). It is not a human disease. Bacteriophages are usually double-stranded DNA viruses. They attach to a bacterium, inject their DNA, and force the bacterial cell to make more phages. NEET asks this to test whether you know viruses can attack bacteria too, not just plants and animals.
RNA mutates faster than DNA because RNA copying has no good proofreading. Viruses with an RNA genome also have a short life cycle, so they multiply and change quickly. This is why a new flu (influenza) vaccine is needed almost every year — the virus keeps evolving. This exact idea was tested in NEET 2023 as an assertion-reason style question.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
The common cold, caused by rhino viruses, and the flu (influenza). NCERT opens the virus topic by reminding you of the ill effects of common cold or flu.
Yes. AIDS is caused by the HIV virus and spreads mainly through infected body fluids such as blood, not through air. It is listed as a viral disease for NEET.
The Tobacco Mosaic Virus (TMV). It was the first virus studied — Dmitri Ivanowsky (1892) linked it to mosaic disease of tobacco, and Beijerinck named the agent 'virus'.
No. Plant viruses usually carry single-stranded RNA. Animal viruses carry single or double-stranded RNA or double-stranded DNA. No virus ever contains both RNA and DNA at the same time.
Cover the mouth while coughing or sneezing, avoid close contact with infected people, and do not share glasses or utensils. This stops droplet (aerosol) spread of viruses like cold, flu and mumps.
