A swab from an infected cut is streaked on nutrient agar. Overnight, round colonies appear. Under the microscope each cell has a wall but no nucleus, and cells divide in two. What kind of agent is this?

An electron micrograph of a nasal sample shows particles about 100 nanometers across, each a protein coat around genetic material, with no cytoplasm. They multiply only in cultured living cells. What kind of agent is this?

A skin scraping from a scaly ring on a child's scalp shows branching threads under the microscope. In the laboratory the sample grows into fuzzy colonies that release spores. What kind of agent is this?

A stool sample from an ill hiker contains oval single cells, each with a nucleus and whipping flagella. They swim, and they live in the gut taking nutrients from the person. What kind of agent is this?

A throat swab from a feverish patient grows nothing in broth or on agar. The same sample makes cells in a tissue culture flask die within two days. What kind of agent best explains this evidence?

A blood smear from a patient with cycles of fever shows organisms with nuclei inside the red blood cells. Nothing grows when the blood is spread on agar. What kind of agent is this?

A pond water sample contains rod-shaped cells that swim. Warmed to 37 degrees Celsius in nutrient broth with no host cells, their number doubles about every 20 minutes. What kind of agent is this?

A doctor removes a flat, ribbon-shaped organism 2 meters long from a patient's intestine. It has repeating body segments and hooks that grip the gut wall. What kind of agent is this?

Lung tissue from a farmer who cleaned a damp barn shows spores that have sprouted into branching threads inside the tissue. What kind of agent is this?

A flu virus enters a cell lining the airway. What does it do that harms the cell?

Tetanus bacteria grow only in a deep wound, yet they cause muscle spasms all over the body. How do they cause harm so far from the wound?

Fluid from a diseased pepper plant is pushed through a filter whose pores stop every cell. The filtered fluid still infects healthy plants. This evidence shows the agent is smaller than any cell.

A boy has bacterial pneumonia. The doctor prescribes a ten-day course of antibiotics. What should the doctor expect, and why?

A girl has ringworm, an itchy ring on her arm caused by a fungus. Her mother has leftover antibiotic tablets from a chest infection and offers them. What should the girl's doctor expect?

A swimmer has athlete's foot between his toes. He is told to apply an antifungal cream for two weeks. What should he expect, and why?

A patient has a tapeworm in his intestine. A clinic offers an antiviral medicine that blocks viruses from copying themselves inside cells. Will it clear the worm?

A boy with a cold caused by a virus feels better on day four and stops the antibiotic he was given by mistake. What best explains why the antibiotic made no difference?

A woman is told to finish all fourteen days of her antibiotic even after she feels well on day five. What is the reason for finishing the course?

A nurse gives students a flu vaccine in October, before flu season. How does the vaccine protect them?

A child has a fever caused by a single-celled parasite carried by mosquitoes. The clinic has antibiotics, an antifungal cream, and an antimalarial drug. Which choice should the doctor make?

A kitchen counter is wiped with a disinfectant that breaks apart proteins and membranes. Which statement about its effect is accurate?

An antibiotic given for a bacterial ear infection will also shorten a case of chickenpox, which is caused by a virus.

In one week, thirty people in a town fall ill with severe diarrhea. Every one of them drank from the same shallow well, which sits downhill from a broken latrine. What is the most likely route of transmission?

Cases of a coughing illness appear in one office. The people who fell ill sat in the same closed meeting room for two hours. Staff who worked outdoors that week stayed well. What route does this pattern support?

Twelve hikers return from a trip through waist-high grass with a spreading rash and fever. Those who wore long trousers tucked into their socks stayed well. What is the most likely route of transmission?

Eight members of a team develop an itchy foot rash. All of them walk barefoot on the same damp locker-room floor. Teammates who wear sandals in the shower are unaffected. What route does this support?

Twenty guests at a wedding become ill about eight hours after the meal. Only those who ate the undercooked pork are sick. Guests who ate the same salad and bread are well. What route does this support?

In a farming village, hookworm cases occur almost only among workers who go barefoot in wet fields fertilized with untreated human waste. Workers in rubber boots rarely fall ill. What is the most likely route?

A day-care center has repeated outbreaks of stomach illness. When staff begin washing hands with soap after every diaper change and before serving food, new cases nearly stop. What route did handwashing interrupt?

A fever appears in one neighborhood only in the weeks after heavy rain leaves standing water in old tires and buckets. Emptying the containers ends the outbreak. What is the most likely route of transmission?

Two students share one water bottle at practice. A pathogen in saliva can pass from one to the other this way.

During an outbreak, cases appear only among people who visited one clinic and received an injection there. Nobody in their households fell ill. What route does this pattern support?

Strep throat is caused by a bacterium and influenza by a virus. Comparing how each one reproduces, which statement is correct?

A mold on damp bread is a fungus and a cold sore agent is a virus. Comparing whether each is made of cells, which statement is correct?

Strep throat is caused by a bacterium and athlete's foot by a fungus. Comparing whether antibiotics act on each, which statement is correct?

Tuberculosis is caused by a bacterium and measles by a virus. Comparing their size under a microscope, which statement is correct?

Giardia is a single-celled parasite and cholera is caused by a bacterium. Comparing their cell structure, which statement is correct?

Bread mold is a fungus and influenza is caused by a virus. Comparing where each can grow, which statement is correct?

A tapeworm is a parasite and salmonella is a bacterium. Comparing how many cells each is built from, which statement is correct?

A vaccine can protect a person against a viral disease before exposure, while an antibiotic is mainly used to treat a bacterial infection that has already started.