How many of us have uttered that depressing phrase this winter? And how many have done the sensible thing and taken two weeks off? Stephanie Northen reports on a dangerous dedication to the job.
For some teachers this winter, the letters a, b and c no longer bring to mind piles of unmarked essays or infants struggling with the alphabet. For them, a, b and c are the three types of myxovirus that cause flu.
Viruses are basically genes in a protein coating, and myxoviruses are sneaky little beasts that alter their coats frequently - medics call it "antigenic drift and shift". This makes it hard for the body to recognise and repel them, and explains why new flu vaccines have to be produced every year. Myxoviruses also happen to make you feel like death warmed up.
This winter's outbreak of flu generated the annual furore about NHS funding. It disrupted schools and sent employment agencies scouring the country for supply staff.
But it also raised questions about attitudes to teachers' health. According to Derek Angood, head of Appleton Roebuck primary in York: "Teachers who have been ill always come back too soon. Or they stagger on and get ill in the holidays when the adrenaline stops flowing."
Last year the absence rate at Mr Angood's school was just 0.59 days per teacher. The reason was obvious when he looked around the staffroom:
"Three-quarters of the teachers were not really fit to be there, but, like the vast majority, they turn up unless they can no longer think, act or speak."
Sue Kaye, head of Merlyn Rees high school in Leeds, was forced to send her key stage 3 children home one day in mid-January. A combination of flu and training courses had depleted her staff. "My teachers are fantastic. They do come back too soon but, to be honest, I'm grateful that they do. This is an inner-city school and our kids need continuity. You can tell when you reach the critical number of supply teachers."
Derek Angood and Sue Kaye are proud of their teachers' willingness to stagger on. But is it really sensible for anyone to return to work before they are fit? Dr David Murfin, a spokesman for the Royal College of General Practitioners, says convalescence has been squeezed out in a management culture that "makes people aware of budgetary pressures and reluctant to take time off".
Bohumil Drasar, professor of bacteriology at the London School of Hygiene and Tropical Medicine, says it takes between one and two weeks from the moment of infection for people to build up an immunity. While teachers would be unlucky to succumb to the same flu virus twice by returning to work too soon, they could be leaving themselves vulnerable to another illness.
Many private-sector companies offer their staff free flu jabs. Teachers are advised against being immunised, even though a jab sold by chemists for pound;20 costs much less than a supply teacher covering for a sick staff member. It is up to the individual and the school, says the DfEE.
But the Government is trying to cut the 2.8 million school days it reckons are lost to teacher absenteeism every year. Late last year heads were urged to come up with procedures to distinguish between "genuine sickness and suspect absence". The teachers' employers' organisation, which rew up the new guidelines, says that absenteeism must be reduced if educational standards are to rise, and the profession is soon to be set tougher targets than the rest of the public sector. Yet teachers take less time off sick than most other public-sector workers, including nurses and prison officers.
Mike Walker of the employers' organisation stresses that they accept that teachers are a "dedicated and committed workforce". The belief is that through better monitoring "we can stop those abusing the system and catch others showing early signs of stress". He adds that "every day of sickness is one too many".
Sue Kaye says she rarely receives sick notes, because it is unusual for her teachers to be off for more than five days in a row. However, Dr Neil Arnott, honorary secretary of the Medical Officers of Schools Association, says a person with flu will be ill for five to eight days and needs up to another five days to convalesce. Like other professionals, "teachers have a high sense of letting people down", he says, and tend to drag themselves into school when they are still recovering from this "very debilitating" ailment.
Olive Forsyth, spokeswoman for the National Union of Teachers, agrees that teachers put pressure on themselves to go back to work too soon. "They are aware of the cost to schools of supply staff and of the disruption to children's education," she says. She also worries that the emotional pressures teachers put themselves under leave them vulnerable to stress-related illnesses.
So are there any advantages in working in such a germ-laden environment as a hospital or a school? Do primary teachers build super-strong immune systems by exposing themselves to the 80 or so infections that children pick up by the time they are five? The jury is still out, but Derek Angood is convinced that new staff are more prone to throat infections than older staff. He puts it down to lack of immunity and being more susceptible to stress. Dr Murfin says there is some evidence that doctors build up immunity over the years, but emphasises that everyone is vulnerable to infection.
There is much research still to be done on such illnesses. Why, for example, will the virus attack five children in a class but leave the other 25 healthy? Professor Drasar says that "we probably all get the virus, but only some of us develop the symptoms". He points to the work of the now-defunct Common Cold Research Institute as an example of the complexity of the problem. For 20 years, the institute stuffed cold viruses up volunteers' noses, dressed them in wet clothing and sent them on long walks in freezing weather. Still only a proportion of people succumbed, and eventually the institute gave up, unable to discern a pattern.
The medical profession also seems reluctant to accept a phenomenon that most employees recognise: that a coldsore throatmigraine will creep up on you at the weekend or just as you are packing your swimsuit and Greek island guidebook. Dr Murfin puts it down to "a change of diet and a change of sleep patterns", adding that time off gives us time to dwell on our problems. Professor Drasar has a simpler explanation. "It's Parkinson's law," he says. "A good executive is always ill on Friday evenings."