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Many times have I heard that anti-vaccine people are dangerous even to the vaccinated population. Is that true? If so, how can it be? People say that germs will attack them, and soon they would eventually grow and spread even toward general population which actually got its vaccines.
I mean it's so counter-intuitive: if I'm vaccinated even when disease will spread I shouldn't be in danger.
Biology is rarely black or white, all or nothing. Protective immunity is generally not an on/off switch, where from the moment you're vaccinated you're infinitely resistant for the rest of your life. You shouldn't expect that, having received a smallpox vaccine, you could have billions of smallpox viruses squirted directly into your lungs and shrug it off without noticing.
Given that (fairly obvious) fact, you should immediately think of scenarios where vaccinated people are still at risk of disease following exposure to unvaccinated people. What about older people who were vaccinated 20 years ago, 50 years ago? What about people whose immune systems are slightly weakened through lack of sleep or obesity or stress? Any of these vaccinated people might well be protected against a brief encounter, but not against, say, being in an airplane seat for 18 hours beside an infected child shedding huge amounts of virus, or caring for their sick child.
It's all sliders, not switches. You can have a slight loss of immunity (4 hours sleep last night) and be protected against everything except a large exposure (your baby got infected and won't rest unless you hold him for 8 hours). You can have a moderate loss of immunity (you were vaccinated twenty years ago) and be protected against most exposures, but you're sitting next to someone on the subway for an hour. You may have a significant loss of immunity (you're a frail 80-year-old) and still be protected against a moderate exposure, but your grandchild is visiting for a week.
Since 2000, in the United States alone, there were 16 reports of outbreaks or groups of outbreaks where the outbreak started with an initial case in an unvaccinated individual and resulted in disease in previously vaccinated individuals. The epidemiology of pertussis is a little different, and transmission happens in many cases apart from a clear outbreak, but there are several documented pertussis outbreaks that led to disease in vaccinated individuals as well. This JAMA report provides a good summary.
Herd immunity is important, especially in highly contagious diseases with no significant non-human reservoir, e.g., measles and pertussis. @iayork's answer is correct that vaccination is not an on/off switch. About 15% of measles cases in the US involve vaccinated individuals, and these have all been the result of transmission that started with an initial unvaccinated case (see previous reference).
These are bad diseases. They can cause serious illness. Half of infants who get pertussis need care in the hospital. For measles, 25% of all cases involve hospitalization. For both, life long disability (deafness, brain damage) and death are real possibilities. 1 in 100 infants with pertussis die, 1-2 in 1000 measles cases die. The harm intentionally unvaccinated people cause to vaccinated people and people who can't get vaccinated (because of their age or other health problems) is real and well documented.
Edit:
In light of another answer and some comments, I'm adding a couple paragraphs putting vaccination in context with other strategies for prevention of infectious disease.
Relevant to this question, as is clear from the epidemiological data discussed above, vaccines are not 100% effective. Some vaccines are more effective than others. Measles vaccine, for example, is in the mid to high 90s. Flu vaccine effectiveness, depending on the population and year, can range from the 40s to the 80s. You can read about vaccine effectiveness in the CDC pink book, Epidemiology and Prevention of Vaccine-Preventable Diseases.
Other strategies for preventing infectious diseases depend on the great diversity of host/pathogen interactions. A few things are generally useful, and often effective. Wash your hands, wear a condom, and stay home when you have a fever. There is no good data on the general advice to stay home when you have a fever, but quarantine is effective for specific illnesses, especially in concert with vaccination and other preventative measures (see the CDC pink book). Limiting the isolation and sedentary behavior associated with staying home to cases where there is a fever has biological plausibility and minimizes the negative immune and other health effects of those behaviors (see Cecil Medicine, Chapter 288).
Along with vaccines, each of the strategies mentioned in the previous paragraph helps protect the individual who uses them as well as other people. So they are good for you and the people around you. You can practice all of them. I'm confused, but not surprised, when the effectiveness of some other strategy is used as an argument against the use of any of the others. You can compare the measured preventative efficacy and effectiveness of each strategy, and come to the conclusion that vaccines generally give the best results (both on an individual and population level), but I'm not sure why, for example, you would decide not to wash your hands because you got a particular vaccine, or not to get vaccinated because you washed your hands.
Something which may help is a model. There is a standard model based on a key number called R₀, the basic reproductive number. This is a highly boiled up number which indicates how many uninfected people will be infected by a single infected individual. Trivially, if R₀>1, the disease will spread through society, and if R₀<1, it will vanish because each infected generation infect fewer new people each generation. R₀ is basically derived from the number of contacts a person can make before showing enough symptoms to sequester them at home, times the chance of any interaction transmitting the infection. The longer the incubation period and the more infectious the disease, the higher R₀ is.
It's a massively massively boiled up model. You can look to science for more accurate models which account for all sorts of features. However, it is a good enough model to demonstrate a concept called herd immunity. Herd immunity decreases the odds of a contact with an infected person spreading the disease. After all, a vaccinated individual is far more likely to go unaffected. This is typically modeled with an extra factor S, such that the reproductive rate of the disease is R₀S. If q is the vaccinated portion of the society, then (1-q) is the number of succeptable people, so the reproductive rate is R₀(1-q).
Now remember, if the reproducitve rate is greater than 1, the disease flourishes. If it's less than 1, it dies off. Having a strong vaccinated base has been shown to be a very effective way of keeping that reproductive rate down.
Consider, as an example, Measles. Measles is considered to be an excellent disease for demonstrating herd immunity. Its R₀ is somewhere between 12-18, meaning each infected person is likely to infect 12-18 people before getting quarantined. If a populus is 95% vaccinated, then the reproductive rate drops to R₀*(1-.95), which is somewhere between .6 and .9. This means the disease will die out if the population is that vaccinated.
Again, modeling based on the basic reproductive number is considered to be a gross tool, but it is effective at demonstrating the risks associated with insufficient vaccinations. If there is a large enough unvaccinated pool to let the disease flourish, everyone is exposed to the disease every day as it runs rampant in the society. Vaccines are awesome, but they are not magic bullets. You can still get diseases after being vaccinated, it's just much more rare (the term to look up is Vaccine Efficacy). The more you're around a disease, the more likely it is that you just get unlucky. Even a vaccinated person benefits from a reproductive rate less than 1, so that the disease never flourishes in the first place. The less effective a vaccine is, the more one is dependent on a high rate of immunization to drive the reproductive rate down below 1.
Also, another strong argument for vaccination is the presence of immunocompromized individuals. Many individuals cannot receive vaccinations for one medical reason or another. For instance, immunizing someone with HIV is a tricky business which I would refer one to a doctor for more information. Those on transplant lists may be advised against receiving vaccinations. These individuals are entirely dependent on the so-called herd immunity effect to insulate them from these diseases. As the reproductive rate for a disease gets closer or above one, it becomes hard to protect these individuals.
Think of every human being as being a walking petri-dish, full of growth medium. When you grow a large colony of micro-organisms, you are allowing that population to experience a larger number of mutations. Every mutation is one more potentional strain that can beat the defenses of our immune systems, even those provided by vaccines. So, if you don't get vaccinated, you are potentially providing safe harbor to the micro-biological parent of the next vaccine-resistant/anti-biotic-resistant super-flu.
This same principle is why large super farms are so dangerous: huge numbers of animals provide more opportunities for micro-organisms to experience mutations that allow them to infect more resistant hosts. The fewer hosts that a micro-organism can infect, the lower the number of opportunities for mutation.
This is one facet of the issue, among those others have mentioned.
