|Broadcast Date||February 18, 2003|
| Previous Episode: Prodigal
Next Episode: Rosetta
Clark rushes to find an antidote after Martha is infected with a toxic virus and falls into a coma, but when Clark also falls ill, it is up to Jonathan to find a cure in time. To further complicate things, Dr. Bryce demands an explanation from Jonathan when he refuses to let her treat Clark.
→ see also Category:Screencaps from episode 2x16
Martha is in the storm cellar burying the can of flour in which she hid the key. As she digs, glowing green dust spores kick up and she inhales them. A few minutes later in the kitchen, she passes out.
Clark and Jonathan rush her to the Smallville Medical Center where Helen Bryce manages her care. She informs them that Martha has inhaled some sort of toxin, but they are waiting on test results to determine what kind and how to treat it. Jonathan asks Clark to run the errands that Martha intended to do that day, which include managing a food drive at the Talon, and Clark reluctantly leaves the hospital. At the Talon, Pete, Chloe, and Lana are sympathetic and concerned, but Clark doesn't want to talk. He leaves abruptly to return to the hospital. While Clark is gone, Dr. Bryce informs Jonathan that Martha is stabilized and announces that she is also pregnant. Jonathan is stunned and informs Clark when he returns. They wonder why Martha didn't tell them.
Lex comes to visit Helen and notices that she has been offered a three-year research fellowship at Johns Hopkins University. She tells him she hasn't decided if she is going to accept it. He encourages her to do what's best for her.
A representative from the Disease Control Agency, Dr. Neil Moore, informs Jonathan and Clark that the toxin to which Martha was exposed is unidentifiable, and that the Kent Farm must therefore be quarantined and searched. Clark superspeeds ahead of them to move the ship. In the cellar, Clark is also exposed to the spores. Clark and Pete drive the ship away from the farm in the back of the Kents' truck. They plan to stash it in Pete's shed, but Clark begins to feel dizzy and drives the truck off the road into a ditch. His strength fails when he tries to push the truck back on the road, but he and Pete finally manage to get it out of a ditch before the DCA drives by on their way to search the farm.
During the search, Dr. Moore and his assistant Dr. Trenton finds the flour can and questions Jonathan and Clark about the key. They feign ignorance and in private, Clark tells Jonathan about his dizziness and loss of strength. Jonathan is worried, but Clark says it is nothing to worry about. The DCA orders a complete cleanup of the storm cellar. That evening, Martha awakens and Jonathan goes to talk to her. Martha confesses that she didn't want to tell him about the pregnancy because she thought saying it out loud would make it go away. When Jonathan asks how it might be possible, Martha tells him that on the night of the tornado, she felt a surge of energy from the ship go through her body. Jonathan concludes that the ship has the power to Heal, enabling her to conceive. Martha further explains that she hid the key from Clark because she was afraid that if he learns more about his origins, he will leave Smallville.
Lana visits Clark at home. He tells her how worried he is about his mother and she tries to reassure him. Suddenly, Clark collapses down the porch stairs. Dr. Bryce arrives to find Clark on the sofa and Jonathan sends Lana home. Dr. Bryce doesn't understand why Clark isn't at the hospital, and to Jonathan's surprise, she is able to draw a blood sample from Clark to examine it.
Jonathan pleads with her to do the testing herself and Helen agrees, promising to keep his medical file confidential. In the lab, Helen looks at Clark's blood under a microscope and is clearly surprised by what she sees. Lex arrives to inquire about the Kents. Helen does her best to protect their privacy, but she can't hide the fact that she is working on something important.
The next day, Chloe comes to visit Clark. She asks to sit with him alone for a while and reads him a letter she has written, confessing that she is in love with him and holds hope that he will one day feel the same way about her, seeing through the 'best friend' disguise. However, Clark mutters, "Lana" in his sleep and Chloe leaves in tears.
Late that night, Jonathan leaves again. Clark is awake and Jonathan tells him that Martha might not make it through the night, but he can't just sit by and lose his family. He has decided to go retrieve the key in the hope that he can activate the ship and heal Clark and Martha. Clark wants to help, but Jonathan instructs him to stay home. Jonathan breaks into the facility where the DCA took the samples from the farm, but a guard corners him. Still very sick, Clark appears and helps him escape. With the help of Pete as a decoy, the two manage to get the key and the ship outside of the hospital. Martha flatlines and is declared dead, just as the key flies out of Clark's hand into the ship and a bright burst of light is emitted — it rolls over Clark and through the hospital and both Martha and Clark are cured. The next day, Martha has made a full recovery and the Kents are overjoyed at the news that their future new arrival is also healthy.
Lex is seen paying Dr. Trenton for a file containing details on Martha's illness. He visits Helen in her office and gives her a key to the mansion, asking her to move in with him and give up the fellowship at Johns Hopkins. She answers him with a kiss.
In the Torch office, Lana and Chloe are getting ready for the food drive at the Talon. When Chloe leaves the room, Lana finds her letter to Clark and reads it. At the food drive, a now-healthy Clark thanks Pete for his help and even though he doesn't remember her visit, he thanks Chloe for coming. When she excuses herself abruptly, Clark asks Lana what's up. Lana cryptically says that seeing Clark sick affected everybody, but some people are better at expressing it. Chloe sees them talking and looks pained.
- Tom Welling as Clark Kent
- Kristin Kreuk as Lana Lang
- Michael Rosenbaum as Lex Luthor
- Sam Jones III as Pete Ross
- Allison Mack as Chloe Sullivan
- John Glover as Lionel Luthor (credit only)
- Annette O'Toole as Martha Kent
- John Schneider as Jonathan Kent
- Dr. Helen Bryce - Emmanuelle Vaugier (3/9)
- Dr. Neil Moore - Michael David Simms
- Dr. Trenton - Todd Talbot
- MP - Sean Millington
- DCA Agent - Craig Veroni
- Steadman - Themselves
- "Not That Simple" - Jamestown
- "Wave Goodbye" - Steadman
- A fever is an abnormal body condition characterized by a high temperature.
- The title refers to sickness that Martha and Clark get after being exposed to kryptonite spores.
- This is the first episode of the series to have no human or even sentient antagonist, just a disease. There had been two similar episodes previously, Nicodemus and Rush; in both those episodes though, the diseases had brainwashed their victims into becoming physical antagonists to fight. The only kind of physical antagonism here comes from members of the DCA, and even here, they're honest people, just trying to do their jobs, which involves saving lives. The only thing that makes them physical antagonists at all is the need to keep Clark's secret a secret.
- Clark uses the following abilities in this episode: super strength and super speed.
- Lionel Luthor does not appear in this episode.
- The Luthor Mansion is not featured in the episode.
- The Talon Mix Pete made is the first Smallville soundtrack, the "Talon Mix" that was released the week after this episode aired.
- The DCA doctor's name (Dr. Neil Moore) is probably an in-joke reference to two famous DC writers—Denny O'Neil and Alan Moore.
- This is the first time Chloe is seen driving her brand new red-orange convertible VW Beetle.
- This episode contains obvious product placement: Clark walks into the Talon with a box of food for the charity drive bearing the Gateway computer logo on the side.
- The Season 2 DVD contains a deleted scene in which Clark and Lex discuss Helen and her impending departure for Johns Hopkins. Clark encourages Lex to do something to keep her.
- Chloe remarks that Clark will end his perfect-attendance streak, but he missed school when he got his heat vision, thinking he was a danger to others.
- Martha's time of death is 2:17 AM.
- This is the first and only episode in the series in which Jonathan has the most screen time.
- In "Fever", time constraints forced the production team to rely on Entity FX to create the Talon marquee that hangs outside of the building, something that they typically do not do. 
- Martha was blasted with energy from the ship in Tempest. She hid the key in the flour can after she found it in Lionel Luthor's office in Insurgence.
- Presumably, Martha's pregnancy is the "secret" that Ryan James asked her about in Ryan.
- Helen was last seen in Visage.
- The blood sample may have been destroyed by Jonathan, but Lex must've kept a sample of Clark's blood to later help engineer the clone and stop the rapid-aging problem that the other clones suffered.
- This is not the only time Clark gets sick. In the Season Six episode Sneeze, Clark gets sick and gains a new power.
- Chloe's letter to Clark later resurfaces almost six years later in Instinct.
- Pete mentions the television show The X-Files which often explored the themes of government-sanctioned investigations and invasions of privacy, like the raid on the Kent Farm. Like Smallville, The X-Files was also filmed in Vancouver, British Columbia, for half of its run. Among other things, the two shows share a visual effects supervisor, musical composer, and numerous guest stars, including Dr. Moore.
- Lex: There's nothing worse than a missed opportunity.
- Clark: It's weird how random memories can just pop into your head, huh? When I was in second grade, I was afraid to go to school because I thought my mom would disappear while I was gone and never come back. I wouldn't even get on the bus in the morning until she promised she'd be there when I got home. Sure enough, every day, there she was when I walked in the door.
- Pete: I'll put the ship between the old Betamax and my dad's Pong game. Indiana Jones couldn't find it down there.
- Chloe: So yet in another classic maneuver to avoid emotional intimacy, I wrote my feelings down so that I could read them to you, thereby avoiding the embarrassing eye contact. (takes a deep breath and starts to read) I want to let you in on a little secret, Clark. I'm not who you think I am. In fact, my disguise is so thin, I'm surprised you haven't seen right through me. I'm the girl of your dreams masquerading as your best friend. Sometimes I want to rip off this façade like I did at the Spring Formal, but I can't because you'll get scared and run away again. So I decided that it's better to live with the lie than expose my true feelings. This is so much easier when you are unconscious. (tenderly caresses Clark's hair) My dad told me there are two types of girls: the ones you grow out of, and the ones you grow into. I really hope I'm the latter. I may not be the one you love today, but I'll let you go for now, hoping one day you'll fly back to me. Because I think you're worth the wait. (again, carefully strokes Clark's brow) Clark? (Clark smiles and stirs in his sleep)
- Clark: (groans) Lana? (Chloe leaves in tears)
- Pete: Clark. X-Files alert!
- Helen: (to Martha) The important thing is that the baby's healthy. And so are you. In medical terms, it's nothing short of a miracle. Your family seems to have a stockpile of them.
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